i thought you said carbon black?
black is by far the coolest colour. it might show up the dirt a little more than the grey, but that gives you the incentive to clean the car and treat it well. remember that the grey will be just as dirty, but since you won't clean it as often, the dirt will stay on your paintwork for longer.... with whatever adverse affects that may have.
black is also best for resale, much easier to sell on.
finally, grey is the colour of john major....
