There is a program called ColorIt Painter that lets you fully colorize Game Boy games. Instead of modifying ROMs, it creates palette scripts that can be read by emulators compatible with it. I'm requesting this to be integrated in VBA-M. Here is the link to the program and emulator integration document. Here and here are the palette scripts so you can try them. I would gladly make a donation if this were to be implemented.
Colorization of Game Boy games
Colorization of Game Boy games
...Wow, what a impressive feature.
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Not sure if this will be in VBA-M, but I do want to experiment with it at least...