brakemeister

Bicycles are painted than they go into an oven and the paint get baked in. If you have a big enough oven and you want to dissamble the whole bike than paint is surely the answer. If you want to fix a scratch than nail polish or hobby paint is the answer.
It works much better than the paint which the manufacturer sometimes provide with the bike. ( If you have some, than after applying it a hot hair dryer might does the job , do not use a heat gun !)

Alternativesly one can also get a spray paint and spray a little bit into the cap, than use a small brusch and use the brush ... Advantage of spray can paint is that it will dry by itself ( allow 24 hours)

for real small scratches a felt pen works great as well ( use the permanent kind)

Thanks Thor