If you don't want to do a paint wash, a fine point paint pen/marker is probably the best way to go. Although, I remember when I first started paneling Gundam models, I didn't have access to gundam markers or any other type of paint pens, so I just used a toothpic and some black model paint; line it, let it set for a bit, wipe off the excess. I guess thats an altenate for those without markers, heh.