For me, that's always meant blacks, grays, and dark "almost black" colors.
Here's an example of a manicure that I absolutely adored. It's rather predictable - blacks and grays, the same as most of the manicures I love on myself. (Yes, of course, you can click for a larger image. Thank you for asking.)
OPI Baby It's Coal Outside, China Glaze OMG, Konad plate M70
Once in a while I'll do something radically different and will be shocked by how much I like it... and this next manicure is an example of that. I did it sometime mid-summer, when I had gotten it in my head that I wanted to do a manicure that reminded me of the beach while I suffered through being trapped in my office building all day every day.
Essie Sag Harbor with Orly Country Club Khaki, konad plate M64
This wound up being one of my favorite manicures of all time. I received more compliments on this manicure than any other I'd ever done. Additionally, it satisfied my need for "beachy" colors. I sighed with happiness whenever I looked at my hands.
Often, in fact, when I try a new color I'll hate it on myself at first. Generally I paint my nails at night, and there have been many times when I'll wind up doing my nails twice in a night or going to bed with a manicure I don't like very much. However, sometimes a new color will grow on me. I'll just need to get used to it a little bit before I'm able to view it objectively, and that will usually take a day or so to happen. My favorite colors are now ones that I hated when I first applied them!
The moral of the story is that we should have fun with our nail polish colors instead of limiting ourselves. If there is one color that you can't seem to break yourself away from (like me and black/gray), try something only moderately different (like a pale blue). Yes, I'm talking to you, all you "OPI Bubble Bath is the only color I'll use" women out there who are scared to try anything new. You might be surprised by how much you like seeing something different on yourself. And after all... well c'mon, folks, it's just nail polish. Not a tattoo!