Why are porphyrins colored when porphyrinogens are colorless?
The precursor porphyrinogen does not have a conjugated pi system that spans the macrocyclic ring. On oxidation to porphyrin, however, the macrocyclic ring becomes fully "conjugated", i.e. connected by sp2 hybridized atoms. The HOMO/LUMO spacing in such systems is much narrower than in non-conjugated systems, which means the photon energy required to excite electrons into the LUMO is lowered from "deep" UV to the visible region, resulting in strong absorbance of light in the visual frequency range.