People need options. But not too many options, or some of them will undoubtedly get pegged as "toy" colors.
Black looks quite good. It should be the standard.
Red could also work. Not very typical for a console, but it's a popular color.
Blue is another typical color, but if Nintendo wants to be "serious", they should make it a dark blue. Possibly one that can be mistaken for black in some conditions.
"Platinum" and "Zelda gold" are very obvious choices which would work very well, but Nintendo should hold off on those ones for special occasions, and stick to the colored-plastic versions for the launch, rather than getting into painted units.
Making one color especially for the girls out there is a very good idea, but you have to be very careful about getting the right color. I think transparent pink, like from the GBA's launch, could work really well. Plus if it's the only transparent color, it'll get more respect, and avoid some of the inevitable childish mockery.
BTW, I don't think the color on the vertical stand is as good as it could be. Something like black might work a lot better, if they're just going to stick with one color for it.
Edit: I forgot white. It's good too. Provided it doesn't get dirty.