In heme synthesis, porphyrinogens are colorless but upon their oxidation (from removing 6 hydrogen atoms), they become colored porphyrins. Why does the change in color occur? Is red the only color it can turn into or are there others (if so, what does that depend on)?
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.