I had the same troubles with my bumper, looked sort of gray colored and
didn't improve much after using the traditional bumper rejuvenators (clear
oily goop).

So I painted with Dupli-Color "Bumper Coat" (Wal-Mart) which turned out
really good only one must be sure to degrease and de-wax the bumper and prep
it (fill and sand if need be) to a good surface first.  Dupli-Color spray
bomb had a nice nozzle on it, not like traditional paint bombs (made novice
painter feel like a pro, I did two coats of primer and two of paint).

After I got this all done I did find on the internet a source for a product
which claimed to bring bumpers back to life when traditional bumper
rejuvenators would not do the trick.  They mumbled something about wax being
the culprit that changed the bumper into that grey dull state that seemed
impossible to rejuvenate.  Unfortunately I can't remember where I saw this
on the internet. (sorry)

The Bumper Coat worked really well for me, Dupli-Color also recommends their
primer to put on first and I suggest that you do all the painting when the
humidity is low.  I then use the traditional Bumper Black stuff (clear oily
goop) over top protect the paint and keep it looking good.  Called
Dupli-Color via their 800 number to find out what to put on top to protect
paint but this number not work from Canada.  Wrote them a few times but no
reply, guess they can't speak Eskimo.

All turned out good enough that I then did my mud flaps, looking toward my
sad looking roof racks next.