On piano hinges that I've painted in the past, the paint ends up cracking and chipping off on the hinge part itself due to repeated mechanical stress. As as for looks, I like shiny stainless and think it looks fine without paint, especially in a boat, even more especially if all the hinges in the boat match. You can buy piano hinge material that is longer than you need, and then cut it down, but then a) they sometimes arrive in the mail bent, and b) after cutting them to length, I find that I'm not mechanically adept enough to give them really nice rounded corners and what not. Because of that (my lack of skill), I like using ones that are pre-made to a length that'll work as-is.
Brina
