GBC games have a "uses GBC features" byte in their header. When run on a GBC, the BIOS detects this byte, sets up the video hardware slightly differently, and passes an identifying value to the game in (I think) CPU registers.
Dual-mode (black cartridge) games use this register value to determine whether to enable GBC-specific engine features. Some games may run at a higher frame rate on a GBC using these features:
- Double speed processing
- CHR HDMA (as opposed to slower PIO copying)
- Faster LCD (as opposed to blur)
Some may show more graphic detail using these features:
- Double CHR RAM
- GBC palettes
- Tile attributes (color, flipping, and an extra bit of tile number)
Some may lock out parts of the game that can't be squeezed into the older hardware:
- Paged work RAM
- Color-specific bonus levels (the color dungeon in Link's Awakening DX and most of Conker's Pocket Tales)
Later in the GBC's life, games became so dependent on these features, especially CHR HDMA, double CHR RAM, double speed, and paged work RAM, that they didn't bother to even try to advance past the legal notice screen on monochrome systems at all. These came in clear cartridges.