The UK had two heatwaves in the span of like four weeks and another one is coming already. As I was melting into my chair like a forgotten wax figurine, my brain turned to some sun/heat related HC for the protoss I headcanon the protoss specifically feed on UV radiation. It's mentioned various times that they feed on light, although seemingly not all light, as the specification tends to be about stars. So there must be something in the light they emit that's special to the protoss that a regular light source can't give them, and my brain gravitated to UV radiation I do think they evolved to process it so much better than humans anyway, since it's their primary source of nutrition and all, so surely they have a sophisticated way to deal with it. But that doesn't mean huge excess of UV light isn't harmful, just like huge excess of food isn't healthy for humans
The Khalai are primarily blue or purple, since these colours are best for absorbing UV light. How mixed and dark their skin is can depend on region and the strength of sunlight it gets. Aiur is hot and sunny, and even their adapted metabolism cannot deal with all the light they get, so protoss naturally evolved scales as extra protection against it. Aiur gets so much sun that it doesn't really matter a portion of it gets reflected off of the scales
But it matters to the Nerazim, who live on a shadow world. Their population is quickly losing scales, so the little sunlight the planet gets can get through their skin with no issue. They're also mostly purple, since it's the best shade to absorb all that delicious UV. Shakuras is also covered in cold deserts and is a rather dry place, so many Nerazim have deep grooves/wrinkles in their skin to better hold onto moisture
The Tal'darim took it in a new direction, their bodies getting rid of a lot of it's pigment, so even more light can get through their skin, because Slayn is similarly shrouded to Shakuras. Slayn isn't much hotter than Shakuras, but is more humid, so Tal'darim skin is smoother than that of the Nerazim. It's also very rare for them to be born with scales now, they simply spent so many generations living on this planet that this adaptation got removed from their gene pool.
Their nerve cords did the same thing. The other two protoss subspecies just have darker cords so UV light gets quickly absorbed by the outer layer of the skin covering them, so the inner layer of the very sensitive organs doesn't get burned and further damaged by radiation. But Slayn gets so little sunlight the Tal'darim began losing pigmentation in them, so their bodies get to have all the tiny little bit of sunrays it can get and nourish them in such a hostile environment. Which works on Slayn, but becomes an issue on planets where sunlight is plentiful, since their cords (and also skin) now lacks the pigment that'd otherwise protect them from excess UV light and they can get burned pretty easily compared to the Khalai or Nerazim
