If the scars are cast on the figure, like some of the Foundry DA types with ritual scarring, it would be quite easy to paint the skin darker around the scar and highlight the raised portion much lighter. That might work. Of course, some scars are raised lumps, whereas others lie in a recess. That is definitely going to alter how they will look.
As for the darker skintones, being an avid tennis fan I have noticed that Serena Williams has a scar from knee surgery. Annoyingly for painters, sometimes it looks much lighter than the surrounding skin, other times much darker. Must be the way scar tissue reflects the light or something. My scars are kind of shiny, and can look different in different light conditions and from different angles.
Couldn't find a decent pic of said knee, but it wasn't through want of trying. You'd think a tennis-player's knee would be in a lot of photos, but those wacky photographers always seem to be trying to capture other parts of the body for some reason...