I have been trying to create realistic looking viens with Gimp basically by brushing a dark blue line and highlighting the sides with dark brown or whatever the skin tone is. It works but doesn't look as good as I want it to. How do you pros do it?
Don't use blue. Maybe use some highlights just a little lighter than the regular skin color. In the nest few days, I'm going to do a skin for Male Hunchback that will have some bulgy veins on the arms; I'll try and remember to do some screen shots.