Like the game is black and white? all games I've had on it are black and white.
Ur wii haz a retro ;)
Check the cables, the plugs may be loose. Broken cables can also cause this. Cables that have been folded and jammed beneath heavy furniture, so that single wires inside the cable may be broken sometimes ruin image transmission.
Check the TVs channel that the wii is on. Sometimes TVs have several video-in channels and not all work with all input devices.
Mine did that for a while. Its just some loose cables, check the back of your tv and the wii, make sure they are all in correctly and it should go back to normal.
Check your cables, make sure they're all plugged in properly, if it's still doing it, try it on another TV.