I paint all my MG kits: 90% airbrush, 5% gundam markers, 5% good ol' paint brush.
The latest fad in Japan is to not paint the kits and make it look like they are, or something like that. Read about it at Dannychoo's blog: No-paint technique
I prefer to paint them 'cuz it lets me hide mistakes (within limits, of course) I might had made during build up.