I've done the paint pen for lug lining, works like a champ.

For the flutes and other engravings, filling in with model paint as others describe. Sometimes I do a wipe right away with paper towel/mineral spirits, other times I wait a while, like maybe a few hours so paint is still soft but not smeary, then sort of scrape excess off w/popsicle stick. This works the best on polished/smooth surfaces and is my preferred option.