I'd go with getting a color that's the same as the color used for the skin, but adjusting it by adding like 20 to the red, and darkening it.
Draw the line where you want the scar with a larger-and-wider-than-the-scar line(on a speperate layer than the actual face), and blurring it with the gaussian(spelling?) at about 4(depending on size of scar), then going with about 50-75+ more red in this line, making it the size of the scar you want, and blur it with about half of what you used for the first blur.
It makes a nice surrounding effect on it.
For an example of what it, after some practice can look like, this link goes to a picture I edited.
Neither scar is real.