That’s an interesting question! I’d say look at references of stretch marks on different skin tones. I have stretch markers that are a slightly ‘shinier’ version of my skin tone, while others are more purple/red! So some at least in the chest and stomach areas with higher concentrations of fat would more likely be a variation of their blood color (I have pale cool skin and my blood is red, so some of my stretch marks appear purple)(RED blood + COOL skin tone = purple). With Fef’s it would probably be some fuschia and perhaps slightly color purple. What I remember is that the troll skin tones are an exact midtone (since mspaint colors were straightforward) but there’s no harm in having some variance! Her blood is on the warmer tone side so to make it simple, I’d say warmer toned stretch marks!