I wouldn't say there's a strict rule to how much contrast you should go for. It's kinda dependent on what the situation requires. I think a basic rule of thumb to follow is the bigger the contrast the more readable your design.
But I would add to that, that there are 3 dimension withing color you can play around with to create interesting contrasts.
You've got Hue:
You've got saturation:
And you've got value:
You can use the contrasts of all three of thee individual dimensions to create a dynamic and readable design.
Edit:
I wanted to add a practical example of my own work. Here's Kyara, each time adding 1 of the 3 dimensions, starting with hue. She's basically just 2 colors, blue and orange.
2nd Edit:
Because we're on the topic of dark skin, I wanted to showcase how I solved it in my own color palette. Since I think that works better for me than trying to do edits. I used the same Kyara drawing and only changed the skin and hair tones. Here's a flat and a rendered version, to show that you don;t need to rely solely on renders to make a readable design.
