One needs only look at the works of Pablo Picasso's "Blue Period" to see evidence of the use of arbitrary color. Since there is nothing in nature that is really blue, Picasso's painting of human flesh as blue is purely arbitrary. Arbitrary color, then, is the use of a color that possesses no natural or realistic relation to the object to which the color is given.
Arbitrary color is often used in Expressionistic paintings in which a color is used to create a mood or suggest emotion or a psychological state rather than depict the object realistically. For instance, Vincent van Gogh's painting, Sunflowers, is an example of the use of arbitrary color. In writing to his brother Theo, van Gogh explained this use, even using the word arbitrary:
"Instead of trying to reproduce exactly what I see before my eyes, I use color more arbitrarily to express myself forcibly."