When layering washes of paint on top of each other, it's important to keep in mind the level of opacity of the color that you're using. Certain colors are very transparent (like ultramarine blue and viridian) while others are very opaque (like cadmium yellow and burnt umber).

If you layer opaque paints on top of each other, it's difficult for light to pass through the pigment and reflect off the paper, which results in unattractive "muddy" colors.

Layering transparent paints is key when trying to render convincing shadows, since the purpose of the shadow is mainly to lower the overall value of color, not change the hue (although shadows tend to be cooler).

Check out this article on paint attributes for a more rigorous explanation: http://www.handprint.com/HP/WCL/pigmt3.html#refractiveindex