Color is being used in cooperation with Line to make colors "pop" more. Along with this, it provides and organized, yet visually appealing aesthetic to the art piece.
Color in particular, is what separates the work into sections that draw the viewer's eyes in, due to the contrast choices by the artist. While Line, physically separates the colors, but provides the attractive composition.
(I would answer principles too, but I don't have the time currently;; Hopefully this helps though.)
