Mario Paint Wii was a canceled Mario game for the Wii. It was only ever shown in a magazine page (which also featured an early version of the game Fortune Street) and seems to be a sequel to the SNES game Mario Paint.
While not much is known, it seemed to have compatability with the U-Draw, a failed Wii accessory that featured a drawing pad. It also seems that the game was nearly complete, as there was much more information then there was for the early version of F.S. on the same page. Here is the description from the ad:
"In Mario Paint Wii, your imagination can take you anywhere - you can draw, color, create music, play games, and even animate your favorite Nintendo stars like Mario, Pikachu, and Yoshi!"
- it is possible that this was a joke put out by the magazine company, but it seems unlikely, as they actually had a license from Nintendo to put only official information in or the license would be terminated.