gameboy colour, is the bigger, old fashioned one, with a smaller screen and the gameboy advance (i assume sp as thats the most current gameboy, although now have the nintendo ds as well but that is very expensive imo) is small, folds in half, comes in funky colours designs, has the small game paks that you can use the older bigger gamepaks on too.
and thats about it really, apart from easier to buy advance games rather than coulour as newer format, although gameboy colour ones cheaper to buy second hand.