From Sega Retro
RGB is a graphics format developed by Sega for use in Sega Saturn games. It consists of a simple 24-bit RGB image (plus a 256 byte header) with the extension .rgb, which can be viewed in a graphics editor which supports loading raw files (such as Paint Shop Pro).
More generally, RGB refers to a color format used by all Sega consoles since the Master System. The Master System uses 6-bit RGB palette (64 colors), the Mega Drive uses a 9-bit RGB palette (512 colors), and the Saturn uses a 24-bit RGB palette (16.78 million colors).