What determines why a specific absorption spectrum is the way it is other than the atomic composition of the substance? For instance, haemoglobin and oxyhaemoglobin only differ by a single oxygen yet that is enough to cause a dip in the absorption spectrum at 575nm. Another example is that despite the fact that biliverdin and bilirubin are pretty much identical in atomic composition, biliverdin is green yet bilirubin is yellow/brown.

Obviously molecular structure and different bonds have to play a role, but I can't find an explanation geared towards laymen as to why this might be, I only found an explanation relating to the energy levels of the different atoms in the substance.

Where can I read more about this? I tried looking around but could not find a good place to start.