If I recall, someone colorized the first level of Marioland 2, but didn't get further than that. I'm confident I could get it working if someone wanted to take care of the actual colorizing work (I'd handle the asm work). This is something I'd be interested in doing, since Mario Land 2 is an awesome game, and I actually have a bit of experience hacking it.
Also, I may try to create an improved version of GB Colorizer that isn't super buggy, or at least publish a document on how we applied the fixes we did to this game. I think the main problem with Colorizer is that it tries to work for every GB game, where really each game draws graphics in a unique way. Currently what it does is insert colorizing code into the Vblank handler, and updates the colors of each tile and sprite every time Vblank happens (every 50 miliseconds or so). It doesn't need to happen nearly this often, which causes a lot of slowdown. Also, it's not very smart about finding what code/data in bank 0 can be replaced, which causes a lot of issues by overwriting important stuff. Ideally, I'd be able to come up with program that can relocate code/data in bank 0 in an intelligent way, but realistically, each game will probably have to be hacked individually with their quirks handled on a case by case basis. So to really colorize a game you'll probably need ASM experience or the help of someone who does.
Anyways, if I start work on a new colorizer I'll post a topic in Personal Projects, no need to derail this thread any further.