It is well known that oil paint darkens and becomes increasingly translucent as it ages. These changes cause visible disfigurement of paintings and, although the phenomenon has been extensively studied, the causes are not definitely known at present.
One way to think of this change is to imagine your painting like a stack of colored glass that with time becomes increasingly darker and more translucent.
This being the case, underlying layers of paint will become increasingly visible, and if they are darker than those of the upper layers, they also acerbate the darkening effect.
These changes can be mitigated by painting over a white ground, insuring that the ground is sufficiently thick and opaque and by building the painting from lighter to darker layers of paint. If it is not possible to build the layers from light to dark, than it is important to apply sufficiently thick and opaque layers, preferably with lead white.