The mednafen multi-system emulator is a great piece of software. The cores that I’ve tried seem accurate enough, the interface is extremely simple and configuration is managed by a simple text flie.
It supports palette files to configure the look of the different cores. I’ve made one for the gameboy that I think replicates the look of the original screen pretty closely.
You can download the palette here, here is a hex dump:
$ xxd ~/.mednafen/palettes/gb.pal
00000000: 9bbc 0f8b ac0f 3062 300f 380f ......0b0.8.
It’s 12 bytes in size, three bytes per colour for four colours:
RGBRGBRGBRGB