When asked what colour is a shadow, the majority of replies would be grey. The human mind fools us into thinking that it must be, but, on closer inspection, the colours can vary through many hues and values.
It takes practice and a good eye to paint shadows, and this skill can create depth and dimension to an otherwise flat piece of work. There are, however, some simple guidelines worth considering to make the process easier.
There are two types of shadow—cast shadow and body shadow. Cast shadow is created when an object blocks the passage of light and body shadow is the area of an object that faces away from the light source.
In both cases, the light will affect the tone of the shadow; if the light is warm, the shadow will be cold and if the light is cold, the shadow will be warm; the more contrasting the light, the deeper and darker the shadow.
To decide which colour your shadow should be, use a colour wheel and find the closest colour match to the object you are painting. Then look across the wheel to the opposite colour; this will be the complementary colour of your chosen match. By adding a small amount of the complementary colour to the original colour, a darker value can be achieved which is totally in harmony with the original.
This is perfect for those not wishing to use greys or black. For example; if you mix black into yellow, you will get a dirty olive green colour, however, if you add deep purple, which is the complementary of yellow, the result is more vibrant with a shadow that looks realistic and tonally correct.
As with all things, it is worth experimenting with varying amounts and different mixes; with practice, your shadows will have been mastered and your paintings enhanced.