If you have an NTSC GameCube and some PAL games will only play in black and white, you can try two things. If you`re getting black and white pictures on an NTSC GameCube console (such as the Panasonic Q), try holding B down when booting and select 60Hz. Some, but not all, games let you do this.

Alternatively get an "RGB cable for a US or Japanese cube", NOT an RGB cable for a European PAL GameCube. Have a look on the net on Cube importers sites. S-video and normal composite cables don`t work.