Watercolor artists often describe a color as staining or non-staining, but what do these terms really mean? At Natural Pigments, we wanted to understand staining more precisely—beyond casual impressions—and to offer data that artists can trust. This article explains how we study staining scientifically while remaining meaningful to painters. It details how we prepare swatches, take measurements, and translate those numbers into the familiar artist labels. We also explore alternative approaches and explain why we chose this quantitative method over others.
Understanding “staining” in watercolor
For artists, staining refers to how strongly a pigment binds to or penetrates the paper fibers and how resistant it is to lifting once dry. High-staining colors are difficult or impossible to remove; low- or non-staining colors can be lifted easily, revealing the paper's white beneath. This usage is consistent across artist literature and manufacturers.
Bruce MacEvoy’s authoritative resource, Handprint, defines staining pigments as those whose particles or dyes embed deeply into paper sizing and fibers, resisting re-wetting and blotting. Winsor & Newton explains the same concept: “Staining colours are those which are difficult to remove once dry; non-staining colours lift easily with a damp brush or sponge.” Daniel Smith likewise classifies its colors from low to high staining, noting that highly staining pigments leave a permanent mark even after repeated lifting. Schmincke’s HORADAM Aquarell line describes staining as color particles penetrating the paper, so they cannot be entirely lifted once dry. All agree that staining means difficulty of lifting.
Historically, artists judged staining by brushing out a color, letting it dry, and then trying to lift a portion with clean water and a soft brush. The results depend on paper type, sizing, drying time, and technique, so qualitative tests vary. Our goal was to preserve that artistic meaning—how much color remains after lifting—while eliminating subjective bias.
Why we developed a numerical system
Visual observation alone cannot reliably compare pigments that differ in depth or hue. A pale green and a dark phthalo blue may appear to lift similarly, but differ significantly in how much color actually remains. To make comparisons fair, we measure relative residual color using a spectrophotometer, following the workflow described below. This approach is more objective yet still reflects the artist’s concept of staining: how much color remains after a standardized lifting process.
We call this value the Residual-Color Index (RCI). It expresses the color remaining after lifting as a percentage of the color in the original wash. The math is simple:
RCI (%) = ΔE(Lift, Paper) ÷ ΔE(Wash, Paper) × 100
Here, ΔE is the color difference in CIE Lab* space measured by the spectrophotometer. An RCI near 0% means the lifted area is close to white paper—non-staining. Near 100%, the lift is almost identical to the original wash—high-staining.
How we prepare our test samples
Our swatches are prepared on Whatman No. 42 filter paper, which provides a consistent, absorbent surface. Filter paper ensures uniform capillary action and minimizes surface texture variations. Each piece measures about 5 × 10 cm. The process follows our internal laboratory procedure for color property evaluation.
We first dilute a measured portion of watercolor paint with distilled water to a fixed paint-to-water ratio. Using a clean 25 mm flat brush, we draw a single, even stroke across the paper at a controlled speed. The wash dries for five minutes before the lifting step begins.
To simulate a controlled lift, we flood a narrow central band with clean water, lay a damp cellulose sponge across it, place a one-kilogram weight on top, and leave it for ten seconds. The sponge is removed and the strip allowed to air-dry. The result is a clear band of lighter color across the wash—identical in preparation for every pigment.
Figure 1 — Preparation of watercolor strips on filter paper. The center band is the area designated for lifting.
Once dry, we record CIE Lab* values for three spots: bare paper, unlifted wash, and lifted area. These readings form the data for calculating the RCI.
Measuring and interpreting results
We compute ΔE—the perceived color difference—between each region. ΔE(Wash, Paper) tells us how far the original wash color is from the paper. ΔE(Lift, Paper) tells us how far the lifted area remains from the paper. Dividing one by the other gives a percentage that represents how much color remains.
For example, if the wash is 80 ΔE units from the paper and the lift is 40 ΔE units away, then RCI = 40 ÷ 80 × 100 = 50%. The lifted area retains half the color difference of the original wash, indicating medium staining.
We assign labels based on the RCI scale:
-
Non-staining: less than 30 %
-
Low staining: 30–55 %
-
Medium staining: 55–75 %
-
High staining: above 75 %
These ranges translate numerical data into terms artists understand. A high-staining pigment resists lifting, showing only a slight change between the wash and lift. A low-staining pigment shows a significant change—the color nearly disappears.
Preparing paper for better lifting tests
Not all papers lift color the same way. Sizing and absorbency are crucial. For artists wanting to prevent staining and lifting, a well-sized surface helps keep water out. We recommend using a traditional natural-resin varnish, such as Rublev Colours Lac Water Varnish, to lightly seal the paper. This thin varnish is brushed on, creating a less absorbent surface that more closely resembles an external-sized watercolor sheet and produces more consistent lifting behavior. When applied evenly and allowed to dry thoroughly, it prevents excessive pigment penetration and makes lifting reproducible across sessions.
Why this numerical method matters
Our method turns a subjective judgment into a repeatable measurement. Because the Residual-Color Index normalizes for the initial strength of the wash, it allows comparison between very different pigments. Artists can see that a pale yellow and a deep blue are both “low-staining” for the same physical reason: both lose most of their color difference relative to paper after lifting.
Manufacturers and artists describe staining qualitatively, but those descriptions rest on the same foundation—resistance to lifting. The RCI expresses it as a ratio. For colors like phthalo blue or quinacridone magenta, the lifted band remains close to the wash, giving an RCI above 75% and a high-staining label. For pigments like cobalt yellow (aureolin) or cerulean blue, the lifted band moves much closer to white paper, yielding an RCI below 30% and a non-staining label.
Figure 2 — Examples of swatches showing Non-staining, Medium, and High-staining behavior after identical lifting cycles.
| Color Name | ΔE(Wash, Paper) | ΔE(Lift, Paper) | ΔE(Wash, Lift) | ΔL | RCI % (sheet) | Category |
|---|---|---|---|---|---|---|
| Purple Ocher | 42.14 | 10.5 | 31.83 | 30.68 | 24.91 % | Non-staining |
| Prussian Blue | 60.55 | 37.23 | 56.98 | 22.49 | 61.47 % | Medium |
| Natural Yellow Oxide | 72.13 | 58.94 | 71.68 | 30.86 | 81.71 % | High-staining |
This system aligns with definitions used by Winsor & Newton, Daniel Smith, and Schmincke, but provides a quantitative scale rather than a visual guess. It’s also reproducible: any lab with the same procedure and instrument can replicate the numbers.
Alternate visual labeling—and why we don’t publish it
During early testing, we explored a “visual” rule that combined absolute residue (the ΔE between the lift and paper) with hue-specific adjustments and changes in lightness (ΔL). This approach produced labels that matched visual impressions for some colors—especially yellows and oranges that appear bright even when most pigment has been removed—but it added subjective judgment. Because different hues appear lighter or darker at the same chroma, this rule cannot guarantee cross-color consistency. We therefore keep it as an internal reference but rely on the RCI scale for published results.
Figure 3 — Numeric (RCI) vs. Visual (Hue-Aware) Labeling illustrates how two different methods for classifying watercolor staining can produce slightly different results.
On the left, the green bars represent pigments ranked purely by their Residual-Color Index (RCI%), the quantitative value derived from color-difference measurements. This scale reflects how much of the original color remains after lifting, independent of hue or brightness. Higher RCI % means more color remaining (high-staining), and lower RCI % means most color has lifted (non-staining).
On the right, the red bars show how the same pigments might be judged using a visual or hue-aware approach, which adjusts classifications based on how the color appears to the eye. For example, yellows or light earth colors may look more intense than their measured RCI suggests, so they might be placed higher (more “staining”) in a purely visual system.
The figure demonstrates that while the visual approach can seem intuitive, it introduces inconsistencies between hues. The numeric (RCI) method, though more technical, provides a fair, repeatable comparison across all colors—showing why Natural Pigments chose the RCI-based system for its published staining ratings.
Why we use this method
This approach bridges science and studio practice. It captures what artists mean by staining—resistance to lifting—without depending on human perception of brightness or color memory. The numbers are not abstractions; they describe what painters see when a wash resists the brush and sponge. At the same time, because they are based on repeatable measurements, they allow us to compare colors precisely and publish data that any artist can verify.
When we say a pigment is High-staining, we mean it in both the artist’s and the scientist’s sense: the lifted color remains close to its original wash, difficult to remove, and rich in chroma. When we call it Non-staining, it truly lifts back toward paper white. The Residual-Color Index puts numbers to those familiar experiences.
References
Daniel Smith. “Watercolor for Beginners – Watercolor Properties.” Daniel Smith Artists’ Materials. Accessed October 2025. https://www.danielsmith.com/tutorials/watercolor-for-beginners/
Daniel Smith. “Making Sense of Staining, Sedimentary and Transparent Pigments.” Daniel Smith Artists’ Materials. Accessed October 2025. https://www.danielsmith.com/tutorials/staining-sedimentary-transparent-pigments/
Daniel Smith. “Watercolor Pigment Characteristics – Staining.” Daniel Smith Artists’ Materials. Accessed October 2025. https://www.danielsmith.com/daniel-smith-watercolor-pigment-characteristics-chart/
Holbein Works Ltd. “Artists’ Watercolor.” Holbein Works Ltd. Accessed October 2025. https://www.holbein.co.jp/en/products/watercolor/artists-watercolor.html.
Holbein Works Ltd. “Artists’ Watercolour – Tubes & Sets (Specification listing includes ‘Staining’ values).” Jackson’s Art (product page). Accessed October 2025. https://www.jacksonsart.com/en-us/holbein-artists-watercolour-paint-15ml-neutral-tint (or raw sienna example).
MacEvoy, Bruce. “What the Ratings Mean: St- (Staining) & VR (Value Range).” Handprint. Accessed October 2025. https://www.handprint.com/HP/WCL/pigmt8.html
MacEvoy, Bruce. “Watercolor Palette: Staining.” Handprint. Accessed October 2025. https://www.handprint.com/HP/WCL/palette2.html#staining.
Maimeri. “MaimeriBlu Superior Watercolors – Staining Colors.” ArtMaterialEssentials.com. Accessed October 2025. https://artmaterialessentials.com/maimeri.html.
Natural Pigments Laboratory. Watercolor Properties Evaluation. Willits, CA: Natural Pigments, 2025.
Natural Pigments. “Rublev Colours Lac Water Varnish.” Accessed October 2025. https://www.naturalpigments.com/lac-water-varnish.html.
Schmincke. “HORADAM Aquarell.” Schmincke Website. Accessed October 2025. https://www.schmincke.de/en/products/horadam-aquarell.
Winsor & Newton, “Professional Watercolour – Composition & Permanence Tables,” includes a note: “In water colour, … colours with [staining properties] cannot be lifted completely with a damp sponge and are marked ‘St’.” https://www.winsornewton.com/pages/professional-watercolour
An educational page on their site (“How to lift watercolour”) states: “Non-staining watercolours are likely to settle on the paper after the water has evaporated… once dried, these colours can be lifted off to reveal the white paper beneath them.” https://www.winsornewton.com/blogs/guides/how-to-lift-watercolour
