KungKras on 01 July 2012
Some carts are only playable on the Colour, old GB games can sometimes use the SNES hardware in the Super Gameboy to display colours, Pokemon is an example of a game that uses the SNES to gain colour, uses the GBC to get colour, and is playable on the original GameBoy.
As for not being two separate systems, I don't know. Maybe there were a lot of GB games released even after the colour update?
I LOVE ICELAND!
