Gray cartridges run in GB mode. The GBC can apply some limited colorization, the firmware has a database of game IDs to apply specific palettes to, or you can select one manually with a button combo on startup.

Black cartridges are dual mode, they'll run in GB mode on original hardware or GBC mode on a GBC.

Translucent green cartridges are GBC-only. If you try to boot them on an original GB you'll get an error screen telling you this game is only for GBC.