I would go with red.

Have you see the pictures of the X6? It looks bad from the rear because of the grey/silver skid plate (or whatever it is) that is has there.

Also, the aero kit is colored to match the car. This skid plate reminds me of that. I would say that your choices are really red or black (but red would still be the first choice).

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2008 X5 4.8i | Wheels: Summer> style 336 20" RFT; Winter> style 214 Ferric Grey 20" RFT | Aero kit | ESS E-Flash tune | V1 | LCI Rear Tail Lights | Coding completed | Engine replaced at 90K