If you just want the casing's color changed, there are companies who can do that for you, for a price. But, as you said, you're lacking funds, but I'm just showing you this if you ever have any extra money...
Now, if you want to open up your Gamecube, You'd need some specific tools. Opening it up also voids your warranty, provided it hasn't expired on its own. Unfortunately, I can't provide any more information, as I have not opened up a Gamecube.