I had an ACS one and painted it body color to match. It looked great.
But on a stock diffuser I would leave it black. Also would leave it black even more so because of the hitch you spoke of. Black would hide the details of the cover more.
OE CSL WHEELS - OE CSL BRAKES - OE AUTOFLD MIRRORS - CSL RACK - CSL TRUNK - CSL DIFFUSER - AA TUNE - AA PULLEYS - AS 40% SSK - BILSTEIN PSS9s
CF CSL LIP - NAV- BMWP CF STRUT BAR - BMW GROUP N MNTS - GRUPPE M CF INTAKE - SUPERSPRINT - UUC S/S LINES, RTABs - H&R SWAYS - 4.10 M-SPORT DIFF