Re: how to paint a plastic gas tank
What color is it? If it is white, you can brighten it up a certain amount with soft scrub with bleach. Otherwise, the only way to color it that I know of that works is to take a permanent marker to it. Even that will scrape off, but it stays much better than any rattle can paint does. They make "plastic dye" in a rattle can--it's marketed to cover up faded plastic on interior door panels in cars--but no word on whether it works on bikes.
Basically though, nothing will be permanent. You may find something that will stick to it for a while but everything out there will scratch after a while. Especially being that it's a gas tank, the fumes that leach through the plastic will destroy everything.