Unlock Geometry Wars:
First, go to the Kudos World Challenge.
From there to the the car select screen.
Then go to your garage. Once in your garage, press Y to walk around freely.
Walk over the the arcade machine and press A.
This will allow you to play the arcade game Geometry Wars, which was created just for PGR 2.

How to change Car Colour:
Go online, and either join/make a game.
Choose the car you wish to change the colour of. (eg Ford Focus)
Choose a car in a different class with the colour you want the first car to be. (eg VX220 for orange)
Go offline, and go to either single player kudos challenge, or ghost trials (the names may be wrong, it's from memory)
Select the class of the car with the colour you want to be changed. (eg the class with the Ford Focus)
Keep pressing A until you see the small 3D version of the car on the right hand side.
Do not select it with A. Once you have seen this screen, keep pressing B until you are back at the title screen.
Join/Create a game on xbox live, and the car you chose (Ford Focus) should be the car you have selected, but in the colour of the other car. (orange from the VX220)
This can be done with any car in the game, but will not "transfer over" stripes onto a car without them