Keep the car original. Unless you are willing to strip the car to do a proper color change it will devalue the car to any prospective buyer at the time of resale. Besides, Sparkling Graphite is the most rare Non Individual M3 color made. You are lucky to have one of the few 08 SG M3's, keep it that way.
I personally wouldn't buy a car that was color changed unless I knew it was stripped down to the bare shell to be repainted.
2011 E90 M3 ZCP - Individual Moonstone/Individual Amarone Extended/Individual Piano Black With Inlay:LINK!!!
1994 Euro E36 M3 Sedan - Daytona Violet/Mulberry:LINK!!!