I tired the heat gun method, but my results didn't last long. Got maybe 3-4 months before it went back to ugly faded grey.
Ended up spraying mine with the Rustoleum bed liner. Did it probably like 2 years ago now and it's holding up well. There are a few spots were it has rubbed or chipped off but its not bad at all, especially considering how rough I treat it. The key with the bed liner on plastic is prep and dry time. I removed the the plastic steps from the bumper to paint them, this way they could dry properly. Remove them and then lightly sand with 220 to rough them up. Wash with dish soap and then degrease them a few times with something like SuperClean or Power Purple. Let them air dry and then do very light coats of bed liner waiting about 30mins between each coat. Once you have a good amount of coats on put them out in the sun somewhere for about a week. This is the most important part. The adhesion to plastic is really bad until it is fully cured and that takes up to 7 days depending on the weather.