There was a time long ago that you could source factory pre-painted bumpers. The primary reason was that the bumpers are already painted separately during the mfg process. Body, hood, trunk and doors are painted together during production. Best bet is to have it done locally while it is attached to the car to look OEM inside and out.
Current - '11 e92 M3 SG/FR ZCP|ZCV|ZP2|2MK|494|415|6NR|752