I'm a little late to the party, but I also would like to know how exactly this was achieved.
I would like to know as well. I might be wrong but I think the following has been done :
Rom has been expanded and 8x8 tiles have been relocated somewhere else to allow to have more space and add palette information (in order to display them in color). Shouldn't be to hard for level tiles because there is a 8x8 tiles pointer in level header that can be easily updated. Now this is only in theory, in practice there is probably lots of ASM hacks to make this work.
Now all that's left to colorize is Super Wario Land: Super Mario Land 3.
Totally agree. This was one of my favorite game when I was a child. I would like to see it in color as well. I like it so much that I made an level editor for it (go to my profile for more info).
I am dreaming of an emulator that would allow to colorize GB games in a simple way. Something like this : you pause the emulation, you see BG map / 8x8 tiles / sprites and you can put color directly on it (eg : flood fill or by replacing any 4 gray color by RGB color). Then you resume emulation and the tiles are automatically put in color based on the rules you selected (like to colorize tile only if surrounded by some other tiles). In the end everything can be saved to a single file that you can distribute easily on the forums or romhacking.net.
Sure, it wouln't work on real hardware like GBC. But at least you could play it on PC. And later someone could make a real rom hack and reuse your colors
Actually this already exists (it's called GB Enhanced
). But I find it cumbersome to use.