Because the game is PAL. PAL video system is different than NTSC (US video system). try to force PAL 60Hz. if you can't then, you'll not able to play the game with colors.
Hey, I'm getting a ISO called "New_Super_Mario_Bros_Wii_PAL_main.dol_Fixed_By_BuZ"
He said that he used [Wii]New_Super_Mario_Bros[PAL][WIISOS.com] and changed the main.dol.
Do you think it will work ?