How are can buoys marked?

This depends what kinda of cans you are talking about? Are you referring to IALA-B, International Association of Lighthouse Authorities Region B Buoyage system (United States) ?
Can buoys for navigation in this region are almost always green & numbered odd. There maybe yellow cans or white, which are special purpose buoys. These either mark a danger area, no wake or special instructions. Yellow buoys may have a unique special purpose.

The opposite of green cans is red nun buoys, which are even numbered. Also you may find Cans or nuns marked with both red and green bands which makes them preferred channel markers.