Refer to www.clearshieldprotection.com which will provide some FAQ's on yellowing, do it your self kits and other pertinent questions regarding paint protection films. Also do a forum search on "Paint Protection films". Hope this helps clear up issues with PPF.
I think you'll find that although the bumper cover is plastic, the color all the way through only works if your car is black. Any deep chip on my silver 2008 reveals the black plastic underneath. Its possible this is different on GenIII but I'd have to see it to believe it. I applied 3M product on my 2002 Solara when new and it hasn't yellowed yet. Car sat in the sun at work for 4 years, garaged since.