The kind of sensors used for line following robots (IR proximity sensors) don't sense color in the normal use of that word. They actually sense reflectivity, the reflectivity of an object at a particular wavelength is what gives it color.
So, your sensor would work best if you had a background that absorbed IR light and a line that reflected IR light. As it turns out, the colors that we call dark colors are pretty good at absorbing near IR as well as the visible spectrum, and the colors that we call bright colors are pretty good at reflecting near IR.