I might say blue... but it'd depend on what you were wearing. Colors are risky in general, because I guess there's always that chance that it would clash with something else in the photo and just look silly.
On a regular day, I'd go for black... but not for pictures. As has been stated, that might look like cavities. :ohmy:
Your best bet, in my opinion, would be silver. Simple. Accommodating. Clean.