Contrast is one of the principles of art which creates excitement and interest in artworks. Two things that are very different have a lot of contrast.
White and black have the greatest contrast. Complementary colors also have high contrast.
Artists use high contrast to make something show up. In Wheatfields with Ravens, Vincent van Gogh used high contrast colors to make the yellow wheat fields stand out against the dark blue sky.
Artists may choose low contrast for a softer look, as Claude Monet did in this painting of a bridge.