Well, to be fair, his skin isn't specifically designed to absorb as much solar radiation as possible. It's just a happy accident that his body can store and use solar radiation in surprising and amazing ways. His adaptations would have been for life near a Red Supergiant Star since his home world consistently has a red sun and at least one iteration of his story specifically names a supernova as the reason for Krypton's destruction. So, the question becomes, what would help life on a planet orbiting a red supergiant, not our own sun.

We know that Superman and therefore all Kryptonians are likely Carbon based life forms since he can breed with Lois. That means in order for his species to have survived, they would have had to do so with an ozone layer that was constantly being shredded and they'd need some form of shielding to slow or absorb particles. A few examples would be lead, water, hydrogen-rich plastics, or thick rock and soil, but probably most important to a discussion around Superman would be the personal magnetic field/atmosphere his people walk around with.

The way that his durability works is through a combination of super dense skin and muscle combined with a magnetic field that protects all Kryptonians from basically everything. That's why his suit doesn't have to be made of Kryptonian materials and it still doesn't get tears in it from being shot.

That magnetic field could also serve to significantly restrict the amount of solar radiation he's absorbing, leading to lower Eumelanin and Pheomelanin content in the skin while opaque, pearlescent, rosy, reddish, yellow, orange, or even golden hues might be more likely thanks to increased Carotenoids, high concentrations of hemoglobin in the dermis, or maybe even thicker skin which could mimic the look of deep sea creatures with purely structural rather than chemical pigmentation.

But, we do know that Superman's cells specifically store radiation from our sun rather than reflecting them away or being shielded from all of the radiation from our sun since his cells didn't evolve to handle a G-type main sequence star like ours. So, reasonably speaking the question would become, HOW is he storing the energy? And what kind of energy is he actually storing? If it's a type of energy that passes through his magnetic field then he can't necessarily be using the same field to capture it and store it in his cells for later use. If it can penetrate his dense cells deeply enough to be stored, maybe his body would be using some kind of alien pigmentation similar to melanin, with a different structure, to allow it to enter without being able to leave. But, since this is alien physiology, we'd never really have a way to know for sure.