The only carbon fibre bits and pieces I have seen, have been on black cars. Several of our cars on the Nurburgring trip last weekend had carbon fibre bonnets and splitters, but they were all black.
They also had cool carbon fibre style BMW badges, where the blue is replaced with a carbon fibre type finish
I would say that a carbon fibre lip spoiler would look, to the untrained eye, as though you couldn't afford to spray it the same colour as the car.