Aluminum G4 15

You can eliminate the graphics processor as the problem by attaching an external display or TV (using the required adapters) and seeing if the image there is OK. Frankly, it's much more probable that you have a display hardware problem, either a bad LVDS cable or a bad LCD display. If you're very lucky, the problem is just a loose connector at one end of the LVDS cable; otherwise, there is no way to determine which is the problem without trying both. The LVDS cable is relatively inexpensive compared to the display upper. The display upper is sufficiently expensive that repair on a unit this old, while perfectly possible, may not be cost-efficient.