You'll need to know quite a bit of programming for this, as far as I can tell (I've never colorized any game).
There is a utility for colorizing games, but most things I've read about it say that it's a piece of shit full of bugs that shouldn't be used, so I'd avoid it.
So... The best way would be to learn how to program for the GBC. Read documentation about the hardware, its ASM language, etc. Then the only utilities you would need are a text editor and any GB assembler you like.
This seems like a good place to start: https://github.com/avivace/awesome-gbdev