Well, priorities. Let's pipedream about games that color would greatly enhance over what the SGB alone can do. Mole Mania works pretty well as a SGB title as it is and Of Myths & Monsters isn't an especially great game. Not suggesting that it wouldn't be great to see them, but I'd put other projects like Mario Land 2, Megaman V or any of the Kirby games ahead of those.
If I could choose any game to put into color it would be Trip World or Roland's Curse 2.
Also as far as easy game to colorize: Boomer's Adventure in ASMIK World, Catrap/Pitman and Chalvo 55. All these games use a really simple and static tileset yet are still really fun AND are not in color on any platform.
Edit: I'd do the colorization if someone else would do the hacking and give me direction on what guidelines to use.