My brother is an artist and he told me his paintings look more natural when he slightly change the hue (color) of the surface near the transition between shadow and light. Here's three examples: (1) without this effect, looks artificial, (2) with this effect, looks natural, (3) with overuse of this effect, looks strange. Assume pure white light (this hue change is only caused by surface properties).
What would be the physical explanation for this effect? The hypothesis I got is that when an object is painted (semi-transparent surface over pure reflective surface), if the rays reach the surface in a near parallel direction, then we get some refraction. If the reflectance of the surface is not a pure wavelength, then we can see some hue drift due to that refraction. Does this make sense? Is there a simpler explanation?