>>714794317
Color correction. It's supposed to make it look closer to how it appeared on the original GBA screen. Developers back then typically accounted for the shitty screen, so they made the art assets super bright and garish to compensate (this is especially noticeable in ports of SNES games). Opinions vary on if this color correction actually looks good or accurate to the original screen, but I like it.

>>713936161
The GBA color correction option in the retroarch mGBA core