Depends on the colour and depends on the girl. Also depends on how good of a job she's done.
I like it when girls keep things simple, and relatively conservative. That means no flashiness, not bright pinks or cheesey reds. Honestly, I don't mind when girls paint their nails black, assuming that they're not doing it in a goth kind of way.
Really, I don't pay much attention to girls fingernails, unless they're really nasty and jagged, or unless they totally screwed up a paint job. Hate to break the truth to girls who spend hours and hours on their nails, but most guys don't really care about that kind of thing.