You would have to mod the game somehow to change the color of the cow. I know it sounds like it should be something simple to do but it is very much not. Maybe you could find a PC game to mod? It would be a lot easier since you control a lot more of the environment running the game. Is this Rayman game on the PC? I know Ubisoft has made some PC games in the past.
Unless you can get a working debugger running on the Wii while its playing the game, you're not going to be able to change the cow's color without altering the data on the disc. And that opens up a big can of worms since you'll need a modded Wii to play a modified copy of a game. That security is probably your biggest hurdle but figuring out the data on the disc well enough to modify an object is easily a close second.
Ubisoft would be the people to talk to, not Nintendo. Good luck. Software development firms are generally made up of a staff working long hours to make tight deadlines. This is especially true for video game development. I just don't see them taking the time to set that up for you. But I guess you'll never know for sure until you ask.