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 AUTOFOLD MIRRORS - CSL TRUNK - CSL DIFFUSER - AA SPORT TUNE - AA PULLEYS - BILSTEIN PSS9s - AS 40% SSK
CF CSL RACE LIP - NAVI - BMW PERF CF STRUT BAR - BMW GROUP N MNTS - GRUPPE M CF INTAKE - UUC SS BRAKELINES, TRANS MNTS, RTABs - 4.10 BMW M-SPORT DIFF