Can You Play Game Boy Games on Game Boy Color? No - Cartridge Shape Different, But GBC Games Work on GB

No. Original Game Boy cartridges do not fit in Game Boy Color—different cartridge shape/connector.

But:

• Game Boy Color games are backward compatible with original Game Boy (play in mono).
• Emulators (PC, phone, modern handhelds) play GB/GBC/GBA ROMs legally if you own the carts.

So you can't plug a gray GB cart into a GBC, but you can play GB games on GBC via emulation or buy GBC versions that work on both.