Painting Skin Tones with Science: Binders, Whites, and Sheen

 Artist Review of Caroline Magnain’s “Modélisation de la couleur de la peau et sa représentation dans les œuvres d’art” (Modeling skin color and its representation in artwork)

The article explains, in practical terms, how skin and paint handle light—and why that matters when painting skin tones. The author Caroline Magnain developed a detailed skin model and validated it using real measurements in “Modélisation de la couleur de la peau et sa représentation dans les œuvres d’art” (Modeling skin color and its representation in artwork) [p. 62]. The collaborators then tested painted panels made with five binders and earth-color mixtures to see how the choice of medium affects gloss, value, and color. The takeaways are directly useful at the easel: choose a binder to set sheen and depth; know when titanium white will flatten differences and when lead white preserves nuance; plan underpainting, scumbles, and glazes that behave predictably; and match your ground and final finish to control glare. This article translates those results into plain, step‑by‑step guidance you can apply right away.

Key Takeaways

• A 22‑layer skin model fits real data. Three parameters primarily drive most color changes: melanin, blood volume, and oxygenation [pp. 59–62].

• The binder sets gloss. Water‑based binders dry matte. Safflower oil dries to a glossy finish [pp. 125–129].

• Color strength depends on the refractive index gap between pigment and binder. Lower‑index binders look lighter at the same load [pp. 136–141].

• Titanium white pushes reflectance toward 1 and maximizes opacity, but can hide binder differences. Lead white, on the other hand, lifts value more gently and preserves surface character in tints [pp. 139–141].

• Kubelka–Munk lets us compare paints fairly by extracting absorption (k) and scattering (s) from reflectance [pp. 113–121].

Figure 1: Layers of human skin—epidermis (with stratum corneum) and dermis over hypodermis. The thin surface envelope (sebum + stratum corneum) produces the small specular highlight; deeper layers scatter and absorb light, shaping hue and value. This layered structure parallels the paint stack discussed in the article.
Credit: OpenStax, Anatomy & Physiology 2e, CC BY 4.0. Source: https://openstax.org/books/anatomy-and-physiology-2e/pages/5-1-layers-of-the-skin

How the research informs artists about painting skin tones

Optical model of skin

The model treats skin as a 22‑layer structure spanning the epidermis, dermis, and hypodermis, with each layer populated by its principal scatterers—melanosomes, keratin, collagen, and red blood cells. Using the auxiliary‑function solution to the radiative transfer equation, the authors fit the model to measured skin spectra and obtained close agreement. They then varied melanin, blood content, and blood oxygenation to observe how each parameter shifts the spectrum and, therefore, the perceived hue and value [p. 62]. For painters, the takeaway is direct: subtle changes in layered content produce predictable shifts on the canvas.

From skin to painted flesh

When painting skin tones, the palette for flesh tones has remained remarkably stable—earth reds and ochres, vermilion, lead white, with green earth for cool shadows—techniques have evolved from encaustic and tempera to drying oils and, more recently, aqueous synthetics. Two paintings that use similar pigments can look very different because the binder and the surface texture it leaves strongly influence the final appearance [pp. 93–102].

Binder experiments

To test these effects, the study prepared controlled panels with five binders—cellulosic size, wax milk, Caparol “acrylic” (a vinyl acetate–ethylene emulsion representative of VAE/PVA wall paints), egg tempera, and safflower oil—and four pigment sets: burnt umber (), red ochre (), and each of those mixed with titanium white. Surface behavior was measured using optical coherence tomography, goniophotometry, and gloss at angles of 20°, 60°, and 75°, while color within the film volume was assessed by diffuse reflectance. The Kubelka–Munk method provided absorption (k) and scattering (s) coefficients, allowing for fair comparisons among films of slightly different thicknesses [pp. 102, 104–121]. These measurements directly inform painting skin tones.

Water in aqueous‑borne binders evaporates quickly. Vinyl acetate–ethylene (VAE) and polyvinyl acetate (PVAc) emulsions (e.g., the Caparol paint used in the study) typically leave more open‑pored, matte surfaces because pigment is less encapsulated. By contrast, artist-grade acrylic polymer dispersions coalesce into a continuous film that envelops pigment particles, slightly leveling the surface texture and producing a visual effect closer to oil, although still less glossy than drying oils. Safflower oil forms a continuous film that levels the surface, so passages appear glossier. As a result, gloss trends highest with oil, lowest with aqueous binders, and is in between with egg tempera [pp. 125–129].

Inside the film volume, binders with a lower refractive index reflect more light, so mixtures appear lighter at the same pigment volume. Oil‑bound films, which have a higher refractive index, tend to look slightly darker at equal loading. The exact ordering still depends on pigment type and final dry thickness [pp. 136–141].

Adding titanium white alters this balance. Because titanium white scatters strongly, it drives overall reflectance upward and can mute differences among binders. Even so, oil‑bound mixtures often remain slightly darker in reflectance [pp. 139–141]. Lead white (basic lead carbonate) behaves differently in tints: its lower refractive index (≈1.9–2.0) compared with titanium dioxide (≈2.7) produces gentler scattering, so mixtures raise value without the chalky flattening common with titanium white. As a result, lead‑white tints usually maintain subtle color shifts and surface character, which helps when modeling flesh and preserving binder effects.

Practical applications for painting skin tones

Choose a binder to fit the passage

• When painting skin tones, keep light areas matte for clear form. Use casein or egg tempera for thin underpainting layers (dead color) beneath oil or in passages that remain independent; casein provides a toothier surface, while egg tempera offers a slightly smoother, tighter film. (Vinyl acetate–ethylene or polyvinyl acetate wall‑paint emulsions, like the Caparol test paint, tend to dry even more matte and open‑pored and are not a substitute for artist paints.) Expect lighter tints at the same load because of the lower refractive index and the rougher surface [pp. 136–141].

• Deep, glossy halftones. Use drying oil when you want depth and flow. Oil reduces surface scatter and raises gloss. It also lowers reflectance, so halftones look richer [pp. 125–129, 136–141].

• Temper the middle. Egg tempera sits between the two. It can be slightly glossier and slightly darker than casein, yet still holds edges [pp. 136–141].

Pair pigments and binders on purpose

• When painting skin tones, earths change more in gloss than in hue across binders. Oil appears smoother and slightly darker. Casein and VAE/PVA emulsions appear lighter and slightly duller at the same load, whereas artist-grade acrylic dispersion typically falls between oil and these aqueous systems [pp. 128–141].

• White mixtures. When painting skin tones, titanium white can mask binder effects and increase glare. Lead white (basic lead carbonate) has a lower refractive index (≈1.9–2.0) than titanium dioxide (≈2.7), so tints rise in value more gently and preserve subtle hue shifts and surface character. When a mixture contains a lot of white, set sheen with the ground’s absorbency (more absorbent gesso → matte; less‑absorbent oil ground → glossier) and with the final finish (matte/satin/gloss varnish—or none), refer to the Safety Notes for guidance on handling and disposal of lead compounds.

Figure 2 (paper Fig. 2.22): Diffuse-reflectance spectra of burnt umber with different binders. Binder choice changes color: diffuse-reflectance spectra of burnt umber for painting skin tones. Oil-bound films exhibit lower reflectance (a deeper tone), while aqueous-bound films appear lighter, confirming how medium selection affects value and sheen.

Build the stack

When painting skin tones, think in layers. Skin and paint are both layered systems, so plan how light will travel through your stack. First block in value with thin, opaque underpainting. Next, establish chroma with lean middle tones. Then adjust hue and surface feel with oil glazes or light‑toned oil scumbles when working over oil layers. Use casein or egg tempera for underpainting layers. Keep layers lean so scattering remains effective [pp. 93–101].

Terminology: scumbles and glazes

• Scumble: a thin, translucent lighter‑colored paint layer brushed over a darker dry passage to lift value and soften edges.

• Glaze: a thin, translucent, darker or more saturated paint layer applied over a lighter dry passage to decrease tonal value or deepen color.
  In this workflow, both are executed in oil over dry oil layers. Aqueous‑borne paints (casein, egg tempera) are reserved for underpainting only.

Science for painting skin tones

• Refractive index. This measures how much a material bends light. A larger gap between pigment and binder means more scatter and lighter tints. Oils have a higher index than water. Water‑based films, therefore, often look lighter at the same pigment volume [pp. 136–141].

• Surface vs. volume scatter. Surface micro‑texture controls gloss. Scatter inside the film sets the reflectance curve that we read as color [pp. 104–113, 128–141].

• Acrylic dispersion vs. VAE/PVA. Artist‑grade acrylic polymer dispersions form films by coalescing polymer spheres around pigment, effectively encapsulating the particles. This reduces surface roughness compared with VAE/PVA emulsions, and the visual result is closer to oil (smoother, slightly darker at equal load) while still drying less glossy than oil. VAE/PVA systems, commonly used in architectural paints, produce more open-pored, matte films.

• Kubelka–Munk (two‑flux). This method utilizes two diffuse light streams: one up and one down. From reflectance, it solves for absorption (k) and scatter (s). This allows you to compare paints without maintaining a constant thickness [pp. 113–121].

• Critical pigment volume concentration. This is the point where just enough binder wets the pigment. Above it, air voids form, surfaces go matte, and color weakens. For most ready‑made tube paints, the pigment volume is already near or slightly below this point. Stay below it for durable films. To achieve matte effects, use casein or egg-tempera underpainting or an oil-compatible matting agent (such as fumed silica, an absorbent ground, or a matte/satin varnish after complete cure) rather than starving the paint and risking underbinding.

• Albedo. The fraction of incoming light a paint layer reflects (by scattering) rather than absorbs; it ranges from 0 (no reflection) to 1 (full reflection). Titanium white raises albedo toward 1, which evens the spectrum and makes different binders look more alike in pale tints [pp. 139–141].

Figure 3 (paper Fig. 2.13): Refractive index of painting binders. Refractive index of painting binders: why oils appear darker and glossier than aqueous films when painting skin tones. The plot compares binder refractive indices across the visible spectrum, explaining lighter tints in low-index films and deeper tones in oils.

Limits of the model and how to read them

Sources of variability in the panels

The panel program isolated binder effects, but some variables are hard to hold constant. Dry film thickness and final pigment volume can change as water, solvents, and oils evaporate or absorb into the ground. Those changes alter micro-roughness and porosity, and appear as a spread in the measurements [pp. 102, 136–141].

What each instrument measures

The instruments emphasize different parts of the film. Gloss at 20°, 60°, and 75° is highly sensitive to slight differences in leveling and surface texture. Optical coherence tomography directly resolves the surface microtopography. Diffuse‑reflectance color measures the combined effects of subsurface scattering and absorption [pp. 104–113, 125–129, 136–141].

Assumptions behind Kubelka–Munk

The Kubelka–Munk approach assumes a uniform, plane‑parallel, sufficiently thick layer under diffuse illumination. When a film is thin, non‑uniform, or strongly forward‑scattering, errors increase. Treat the absorption (k) and scattering (s) values as comparative trend indicators rather than absolute numbers [pp. 113–121].

Pigments and layer design

The pigment set was limited to burnt umber, red ochre, and their mixtures with titanium white. Other pigments—especially organic lakes or high‑index blues—may show different degrees of change across binders [p. 102]. Historical flesh is also a multilayer system, comprising ground, opaque underpaint, translucent glazes, and often varnish. Those additional layers modify both surface and volume scattering beyond what a single‑layer panel can capture [pp. 93–101].

How to apply the trends

When painting skin tones, read the findings as directional. Oil‑bound films usually present higher gloss and slightly lower reflectance; casein or acrylic dispersion tends to dry matte and lighter; egg tempera sits between. Expect the ordering to hold, although exact numbers may vary depending on your ground, thickness, and application method. Always test your mixtures on your ground under your lights before committing to a larger passage [pp. 125–141].

Studio guidance for painting skin tones (step‑by‑step)

1. Decide the sheen for each area. For light passages in the underpainting stage, use casein or egg tempera for thin, matte layers; these keep planes readable. Reserve scumbles (light over dark) and glazes (dark over light) for the oil‑over‑oil stage. Do not apply casein or egg tempera over dried oil layers. For halftones and accents, use oil or a lean resin‑rich medium to build depth and a controlled gloss [pp. 125–129].

2. Pre‑mix two sets of color. For every flesh hue, prepare an oil‑bound version and a casein‑ or egg tempera‑bound version. Test both on your actual ground under D65 standard daylight (~6500 K, the reference light used in color measurement) and under the room lighting where the work will be viewed.

3. Keep mixtures lean, especially in the underlayers. In the block‑in, favor pigment over binder so films stay thin and dry quickly. For later passes, adjust handling with a small amount of medium rather than adding a lot of fresh binder. Avoid solvent‑only washes that can leave underbound pigment, especially since tube oils are already formulated near the critical pigment volume concentration.

4. Use white with restraint. Titanium white is excellent for maximum opacity and a cool, bright (slightly bluish-white) tone, but it can also mask binder differences and increase glare. For tints that rise in value more gently and preserve surface character, consider lead white (basic lead carbonate); its lower refractive index than titanium dioxide keeps subtle hue shifts visible. When you need to raise value without flattening surface character, add small amounts of translucent earths (for example, yellow ochre or Venetian red), or use lead white judiciously, instead of piling on titanium white [pp. 139–141]. Refer to the Safety Notes for guidance on handling and disposal of lead compounds.

5. Plan the layer sequence (“the stack”). First, set the main values with thin, opaque underpainting. Next, place middle tones in thin layers. Where you need to raise value locally over a darker dry passage, use a light‑toned oil scumble; where you need to deepen value or enrich color over a lighter dry passage, use a lean oil glaze [pp. 93–101].

6. Control leveling and texture. Allow oil passages to reach tack‑free before applying lean oil scumbles on top; this keeps the underlying texture intact and helps prevent unwanted sinking.

7. Choose the final finish deliberately. Casein or egg tempera underpainting layers work well beneath oil passages or in self‑contained areas. If you intend to varnish, make a test panel first: varnish fills texture and increases gloss, which may change local value.

8. Watch pigment volume concentration. With most commercial tube paints, the pigment volume is already near or slightly below the critical level. Do not attempt to create matte passages by starving the paint with solvent or by adding excessive amounts of extender. If an area looks chalky or fragile, add a small amount of binder or apply a thin oil glaze to restore cohesion. For a matte appearance over oil paint, use oil-compatible methods: add a small amount of fumed silica or a manufacturer-approved matting medium to the oil paint, adjust the ground to a lower sheen, or choose a matte varnish after the paint has fully cured. Use casein or egg tempera only beneath oil layers, not over them.

Ground, Paint, and Final Picture Finish: A Sheen Matrix

Use this quick matrix to plan surface sheen when painting skin tones. Read across each row as a complete recipe; test on a small panel before committing.

How to use this matrix: Choose the ground to set your baseline sheen, adjust in the paint layer (lead vs titanium; optional matting within oil), then lock it in with the final finish.

Lead White versus Titanium White in Tints

Use this quick selector when choosing a white for painting skin tones and highlights.

Note: Lead compounds require careful handling and disposal—see Safety notes below.

Safety notes

• Lead white and vermilion. Avoid dust. Wear gloves. Keep food away. Rags with oil can self‑heat; dry them flat or store them in a can with water. Follow local disposal rules.

• Acrylic dispersion and casein. Low hazard in normal use. In‑can preservatives and ammonia (present in some acrylics and in certain casein make‑ups) can off‑gas. Ventilate and wear nitrile gloves.

• Egg tempera. Mix fresh. Keep cold if held for a short time. Discard at day’s end.

Comparison: binder behavior with earth pigments

Trends from panels with burnt umber, red ochre, and their titanium mixtures [pp. 125–141].

The science of painting skin tones for artists

Think of painting skin tones as building an optical stack. Choose the binder to set gloss and depth rather than adding more pigments later to force the result. Use titanium white sparingly when you want the surface texture to remain evident. Always test on your ground under your lights; small changes in surface roughness or refractive index can alter the look as much as a color change.

Bibliography

Magnain, Caroline, Mady Elias, and Jean‑Marc Frigerio. “Skin color modeling using the radiative transfer equation solved by the auxiliary function method.” Journal of the Optical Society of America A 24, no. 8 (2007): 2196–2205. https://doi.org/10.1364/JOSAA.24.002196.
Annotation: Builds a 22‑layer skin model and checks it against measured spectra. For artists, it reveals which parts of the skin drive color and why thin, stacked layers yield lifelike results.

Magnain, Caroline, Mady Elias, and Jean‑Marc Frigerio. “Skin color modeling using the radiative transfer equation solved by the auxiliary function method: Inverse problem.” Journal of the Optical Society of America A 25, no. 7 (2008): 1737–1743. https://doi.org/10.1364/JOSAA.25.001737.
Annotation: Finds a small set of factors that can explain a skin spectrum and recovers them from a measurement. This supports targeted changes in paint—value, chroma, and translucency—rather than guesswork.

Magnain, Caroline. Modélisation de la couleur de la peau et sa représentation dans les œuvres d’art. PhD thesis, Université Pierre et Marie Curie – Paris 6, 2009. https://theses.fr/2009PA066501.
Annotation: Pairs the skin model with tests on painted panels using five binders and four pigment sets. The binder trends in gloss and lightness map cleanly to oil, tempera, and water‑borne practice in the studio.

Binder Reference (Typical Dry‑Film Refractive Index)

Note: Values are approximate for transparent films at a wavelength of 589 nm (sodium D). Paint films include pigments and fillers that modify effective RI; emulsions evolve as water/solvent evaporates.

Glossary: Optics Terms for Painters

Couch (optional intermediate layer). A very thin, wiped film of bodied linseed oil used on a dry underlayer to even absorbency and improve leveling before glazing. Preferred to resin interlayers; apply sparingly.

Critical pigment volume concentration (CPVC). The PVC at which there is just enough binder to fill voids between particles.

• Below CPVC: films are denser, glossier, and stronger.
• Above CPVC: films become more porous and matte with reduced strength; appearance can shift even if PVC is unchanged.

Diffuse reflectance (hemispherical). Light reflected in many directions, captured with an integrating‑sphere spectrophotometer. In SCE (specular-excluded) mode, it includes both surface-diffuse (from microtexture) and subsurface/body-diffuse (from scattering within the film).

Holdout. The ability of a paint layer or ground to prevent the binder from soaking in. Good holdout keeps more binder at the surface, improving leveling and specular reflection without adding excessive free oil to the paint film.

Integrating sphere. A coated hollow sphere that collects reflected light from all directions to report hemispherical quantities. SCI = specular included; SCE = specular excluded.

Kubelka–Munk (K, S). A two‑flux model that relates diffuse reflectance to absorption (K) and scattering (S) coefficients of a paint layer.

Pigment volume concentration (PVC). Volume percentage of pigment + extenders in the total non‑volatile portion of paint: pigment ÷ (pigment + binder). Solvent and driers excluded.

Refractive index (RI). How strongly a medium bends light. High‑RI pigments in lower‑RI binders increase scattering; lower contrast reduces scattering and deepens appearance.

SCI/SCE. Instrument settings for integrating‑sphere measurements: SCI collects both diffuse and specular; SCE excludes the specular component to emphasize the diffuse lobe.

Specular reflection (gloss). Mirror‑like reflection from the paint surface; responsible for highlights. Measured with gloss meters or angle‑resolved setups.

Subsurface (body) scattering. Scattering that occurs within the paint layer as light interacts with pigment particles and pores.

Surface‑diffuse scattering. Diffuse reflection caused by microtexture at the paint surface that breaks up the mirror component.

Transmittance. Light that passes through a substance, such as a paint film.

Varnish (final topcoat). A removable protective coating applied after adequate curing. Not a painting medium; do not add varnish resins (e.g., damar/Maroger) into the oil paint film.

Viewing‑angle effects (angle‑dependent reflectance). Changes in appearance with observation angle, governed by the bidirectional distribution of reflected light (specular vs. diffuse) and surface micro‑roughness; measured with angle‑resolved geometries.