Black primer is how MB supplies their replacement fenders. See this(even though it's not what you're after):
http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=2422531888&category=42611
Body shops spot match a fender ALL the time.
A lot of people prefer OEM to after market,, but there is a HUGE price differential. I used MB on mine, but have had success on other cars with aftermarket. Wreckers yard fenders sometimes have a "set" to them that doesn't coincide with the peculiarities of another vehicle and as an amateur I've had a problem in the past with used (and one very nice success!)
Hanging a fender is not just bolting it on. First, put it on with the bolts finger tight. Then, align the hood/fender seam (because it's most noticable) and tighten the upper bolts/screws a bit and work down. Once at the bottom, go back to the top and door seam and loosen a few bolts/screws and do it again. You have to kind of "massage" it into the best position repeatedly. A little controlled force helps, having the fender bolted up in one area while working another spot into position, then snugging up the bolts/screws in the new area.
Don't dump the old fender's hardware (screws, shims, rubber bits, wheel well lining, lower fender cladding), because your new fender will be bare when you get it.