Supposedly Superman gets his powers from absorbing sunlight. I've read that plants on planets with dim red dwarf suns would likely appear black to our eyes, absorbing across the entire visible wavelength range in order to use as much of the available light as possible. Krypton had a red sun, so if Kryptonians evolved a photosynthetic ability, shouldn't Superman's skin be black?
I don't believe it is ever stated that Kryptonians are photosynthetic; only that the exposure to the radiation of a yellow sun triggers enhanced abilities.
Even if they were photosynthetic, it wouldn't necessarily entail a pigment change to the degree you're discussing. Kryptonians eat and drink as humans do; plants, generally speaking, do not.
Kryptonians therefore would not require special pigmentation to use as much of the available light as possible, because they would not be nearly as dependant upon light as a source of nutrients as plants would be.
Disclaimer: There is no good answer to this question.
Inference: Not likely. Since the nature of how plants and animals derive energy from sunlight is very different, the Kryptonians being animals, would NOT necessarily need to be dark-pigmented since they derive their energy from eating other plants and animals, not from the Kryptonian sun itself.
The Kryptonians are a literary creation. As such, they were created by writers who projected their ideas into a mythical mode in order to tell stories.
- There is no indication in canon that the Kryptonians were able to utilize red sun radiation in any way to alter their metabolism. The original stories told a tale of super-humans, whose powers were derived from superior strength and the gravity of the larger than Earth planet Krypton. These were genetically enhanced supermen whose abilities were theorized to one day be possible (in humanity).
Unfortunately due to writing creep, Superman's powers continued to expand and evolve, and eventually the element of the yellow rays of the sun found their way into the legend.
The powers derived by the yellow sun are never fully explained and without an explanation, based somewhat in science, it is impossible to determine how or why the Kryptonian powers function AT ALL.
Given the surface area of a Kryptonian, even if he were able to absorb all the radiation from the sun across his entire body, 24 hours a day, he would not derive enough energy to be able to perform even one of his super-feats. There is still a missing element to the conversion or translation of solar energy once it is metabolized by the Kryptonian.
It is likely this element or capacity will never be discussed due to the intrinsic story-telling problems likely to occur.
The colors that we see are due to the reflected electromagnetic radiation in the visible portion of the electromagnetic spectrum. These are photons with wavelengths between 400 and 700 nm. If he was not absorbing in this range, then he would appear normally colored to us.
The photons of visible light are not the highest energy photons coming out of the Sun, so it is possible and that he would absorb higher energy, shorter wavelength photons (e.g., UV light) that are invisible to us anyway.
Interestingly, plants and our eyes do not use these higher energy photons because they have too much energy and they cause damage to the cellular mechanisms that would be used to harvest them. There are some animals (e.g., bees) that can see a little bit into the UV. In fact some fish can see UV because their prey have UV protective pigments (i.e., natural sunscreen) so they show up as dark in UV light.