So I've been having problems with my PAL Gamecube being too bright. I've used my Google-Fu to find a fix for it but I've only come across things related to other Nintendo consoles (N64 and SNES).
From all the pieces I've gathered around the internet I've found a solution to this problem.
It happens when you use an NTSC version of the Gamecube A/V Composite cable on a PAL console.
I've done the following and it worked pretty good.
You will need the following things for this mod:
1) an A/V composite cable (Which you can buy from ebay for like 3$)
2) a soldering iron
3) a 75ohm resistor
4) a hammer
5) a bit of soldering tin
Basically you solder the resistor from PIN #3 to PIN #7 and #8. The resistor lowers the brightness of the picture making it normal as it should be!
Take the hammer and gently brake apart the plastic around the cables input side (the side you connect to the Gamecube).
I haven't done this on an official cable because it would be a shame to wreak one, just buy any one of those cables they sell on ebay for a few dollars.
Solder the resistor to the #3 PIN (the yellow cable) and the other side to the #7 and #8 PINS.
And that's it!
This site helped me alot!