Question: **The context:**

There is a population of people surviving on a lunar-analogue's surface, descended from the crew of a crashed spaceship. The rest of their society is not relevant to the question, but their technology includes cobbled together habitats and void suits that protect against some solar radiation but not all (an arbitrary amount that allows them to maintain a population, but not necessarily easily).

**The question:**

What skin colour would this select for? I'd initially say a pallid white given the lack of UV exposure, but I've recently stumbled upon research which suggest melanin provides at least some protection against gamma radiation: <https://www.news-medical.net/amp/news/20110824/Melanin-also-protects-from-ionizing-radiation.aspx>

The question is, what skintone would partial protection from gamma radiation on a longstanding permanent lunar culture select for?

For reference, this is for an art project where colour palette will be important, so injecting some realism into the skintone and working from there would be the way to go.

Answer: 
It won't differ much from the mixture available in the founders' pool. 

The reason is simple: while when our ancestors moved out from Africa to colonize the world had the pressure resulting from lower UV exposure that allowed for the selection of paler skins, I am pretty sure your colonist would be assuming integration vitamin D, removing any need for the body to adapt.

In the case they would not be assuming vitamin D integrators they would still keep the original mix for quite some time: it takes lots of generations for a character to spread, and humans are not that fast breeders.
A detail that heavily effects the answer is what temperature is maintained in the colony. Several complex factors will effect temperature, and you can basically choose whatever fits the story. Temperature will effect how much clothing is worn. Amount of clothing then effects melanin levels. 

* hot->little clothing->high melanin to shield from uv
* cold->thick clothing->low melanin to allow vitamin D production in the little exposed skin

If you are assuming sufficient clothing to block UV, then then assumption that gamma radiation will have the dominating effect is flawed. If opaque clothing has a negligible effect on blocking gamma rays, then so will opaque skin. Since high energy gamma rays penetrate opaque clothing they will penetrate skin as well. Clothing will actually perform better than skin could. Based on the description of the environment, it seems likely that clothing will be made from animal and plant tissue. Many animals and plants will adapt to the environment faster then humans due to shorter life cycles and selective breeding. This means clothing will more quickly adapt to blocking any radiation that can be blocked than humans will.

There is one way melanin might have a greater shielding effect than clothing. If a much thicker layer of opaque tissue could be used for shielding than just the skin. This could lead to the possibility of pale semi translucent skin for vitamin D with melanin rich fat, muscle, and or bone tissue. It sounds like you are wanting a very striking look and a way to justify it, this combination may be fitting.