The colors underneath the main paint layer that serve to modify the paint colors in some way. This can be as simple as a pure white ground that increases the brilliance of transparent colors above. A colored ground provides a mid tone to judge colors against, and also can help unify a picture where it shows through here and there. The most sophisticated colored grounds have different colors here and there for effect. A yellow may be under a transparent blue to produce the effect of green, a blue under crimson for violet and so on. A different tone of the same color might be used with the top layer painted in a rough and patchy manner. A neutral complementary can work well in this case too. The use of underpainting can be an important creative tool for the artist.