I had my exhaust tips powdercoated flat black a while back - and I've been obsessing about how much I dislike the bare-metal muffler. I went on a painting spree today (my wife's Jeep got a freshly painted bumper step and a flat-black muffler and tailpipe), then the Boxster went up on the ramps and got the flat-black muffler treatment.
Autozone carries high-temp porcelain spray in a rattle can - I spent about six bucks on the paint and used about $40 worth of painter's tape to mask off the Boxster.
It's a little hard to make out in the pic - but the muffler is now a much more neutral color and makes one a lot less likely to concentrate on what's behind the diffuser.