When the Game Boy Color was launched by Nintendo in 1998, it had the ability to use cartridges (‘Game Paks’) from the original Game Boy, albeit in monochrome. Its own cartridges, usually translucent, were not backward compatible with the Game Boy but there was a third type of cartridge introduced at the same time as the Game Boy Color, and this was compatible with both models.
These ‘dual mode’ (also known as class B) Game Paks were usually black, and were the same shape as the original Game Boy cartridges (that were usually grey) including the notch that allowed the power switch to be moved across. They were programmed to play in colour when used on the Game Boy Color.
The dual mode Game Paks were compatible with all Game Boy models, except for the Game Boy Micro.