Enamel paint from model shops works well for touching up. I always paint it on carefully then leave it a few days to dry but not fully harden. Then, because it hasn't fully hardened, you can actually flat it back with a light cutting polish and a bit of kitchen roll. The colour then looks a bit matt, but leave it a week and give it another polish and it will come up nice and shiny. When I do this I always think "Wow, I've made a wicked job of that. If I'd have just got the colour match right it would have been almost un-noticable"