Oxidized FAD is a conjugated molecule that absorbs light at 460 nm (Blue). Because the maximal absorption for oxidized FAD is at 460 nm, its color in solution will be complementary to the wavelength of maximal absorption. E.g. because the wavelength of maximal absorption is 460=blue, the color in solution is complementary to blue, which is yellow (Use a color wheel). The reason it goes to color less is that when FAD is reduced, it gains hydrogens and the Nitrogens convert from sp2 to sp3; which reduces the conjugation and consequently causes the molecule to no longer absorb light in the visible spectrum.