Ive had my cossie for nearly 2 years now. Ive noticed the drivers door is slightly darker the the front wing and the passenger door. The colour is diamond white. Can i use a cutting compound like G3 to get the colour a bit lighter.

 

You could try this.
I've got a white one and i've just painted a couple of bits on it so the rest of the car stood out a mile. So i buffed the rest of the car and what a difference.
The trouble with white is there's loads of different shades so if it's been painted it might not have been a perfect match.
I really struggled with mine, sometimes if you look really closly you can see a slight difference.
Try the G3 though on the rest but you'll prob end up doing the rest of the car like me.

 

i got my car painted over here and the painter matched it 100%.. but yh you can try g3 but it wont be 100% even then.

 

Yeah you can see a bit of difference when you really look at mine but not half as much as before it was buffed. It's just a nightmare job but was worth it.