Da Boss wrote:
There are brown, purple and red leaves which photosynthesize here on Earth. I don't think it's too big of a stretch to imagine Orks who have differing algal pigmentation that results in different skin colours given the size of the galaxy and the general diversity of the Orks.
, all plants use chlorophyll for photosynthesis, which is green, so the colouration in such plants comes from other pigments (similar to the reddish orks on Golgotha, funnily enough).
However, there are a number of algae that photosynthesis with much rarer red-to-purple pigments that are similar to the pigments that allow eyes to detect light. The name of the pigments escapes me at present (I want to call them phytoretinols, but that may have been pulled out my arse). So it is feasible to have an entire subspecies of red-purple orks that photosynthesisr with a different pigment. You could give them backstory as the descendants of an early experiment of the brain boyz or something.
Edit: found it. I was indeed pulling the name above from my arse. The molecule is called bacteriorhodopsin. https://en.m.wikipedia.org/wiki/Bacteriorhodopsin