Well, their primary pigment is obviously red, although the exact shade of red varies a bit between individuals. Aside from that we don't know exactly how it works.

We know some imp have black pigmentation spots, often in the form of black rings and spots around their tails like Moxxie and Blitzo has. Imps also tend to have black-looking eyelids, which generally comes across as natural, rather than any kind of makeup, although this is not portrayed consistently whatsoever, Blitzo and Moxxies eyelids are often shown to be black, other times not.

There's also a question of if imps can have more unusual black markings naturally(due to their demonic nature), or if such more complex markings are artificial somehow. Mainly, are Blitzo's clown-stripes on his eyelids natural, are they make-up, or are they tattooed or otherwise created deliberately? Same questions about the marking on Blitzo's forehead.

Then there are the white markings, imps commonly have large, patchy blotches of white pigmentation on various parts of their bodies. Likely, this is primarily just a natural type of pigmentation pattern for them like the spots on a cow, since it's very common, and also seen in imp children. However, the old poster of Blitzo shows him not having the big white blotch on the side of his face, so if the poster is accurate, it means he used to not have it. This either suggests that imps can at least sometimes acquire new white spots naturally as they age, or it could mean he got some of his spots through some kind of incident, perhaps they are markings left by some kind of injury in his past.

The fact that Sallie May has a white mark that resembles a scar across her eye, and Millie has what looks like scars on her legs suggest that white markings can be a sign of imp scar tissue, which would fit with the idea of Blitzo's half-white face being a sign of some kind of injury, like a burn mark.

Also, white pigmentation can also come in smaller, more freckle-like varieties, like on Moxxie's cheeks.

So yeah, that sums up what I can put together about imp skin. If you wanna design an imp, start with a base red, then usually you'd add some large white markings or freckles, bonus points if you use them to indicate scarring. Then you can also add some black spots if you like, usually on the tail.

I'd say base colour of skin is varying shades of red with white splodges and spots placed about and some have black rings on their tail

Okay ty @Lazy Skeptic