From these equations it is easily verified that I o + I e = 1 for all ϕ ϕ , so that the colors are complementary. Without the phase-inversion rule, there would be a plus sign in front of the last term in the second equation, so that the ϕ ϕ -dependent term would be the same in both equations, implying (incorrectly) that the colors were of the same hue.