I don't know if they are commercially available anymore, but I used to sometimes wear blue to dark blue (like a navy blue) tights in class. Citibob already mention gray, which is a good alternative to black.
Some guys even wear white tights, but I would not put this color at the top of my list, unless you're very comfortable with it.
I've also seen men wear spanex/lyrca trunks (particularly in the Summer when studios can be quite hot and steamy) and socks and ballet slippers, and sometimes without the socks. And then maybe a V-neck T-shirt on top or a leotard.
Personally, I like to wear loose-fitting workout leggings (usually sweats made out of cotton and thin) and a long T- or Polo shirt. Thin, wool socks and ballet slippers. I find that unless it's really warm, the bottoms of my feel suck the cold of the floor right up if I'm wear a cotton product. Not true on wooden floors but on all others.
Best wishes for happy and nattily-dressed dancing!