A relatively simple program that detects which model of Game Boy it's running on and prints the initial values of the registers that were used to determine this.
RGBDS is needed to build, make sure its tools are in $PATH and run make.
Two ROMs will be built, a .gb and a .gbc, only difference between them is the GBC compatible flag being set in the ROM header. The color-incompatible one can be used to test the DMG compatible mode on the GBC/GBA, which has different initial register values from the color-compatible mode.
If you see red text, this means your emulator is not locking out accesses to the GBC palettes when it's supposed to.