I just got done painting the fender on my KX. I went from green to black and put 3 coats on of gloss black. I sprayed them down with de-greaser and washed for about 20 minutes before I painted them and the paint has already come off. Mostly from just trying to slide them on but I could see a branch or something easily removing the paint.
The metal gas tank(if sanded down or already unpainted) should work good however. Paint isn't very strong on plastic.
Krylon makes a spray called Fusion for plastic. I havent tried it. I always use Duplicolor for metal. It has a very nice spray pattern and is pretty run resistant. Just make sure you remove ALL the old paint, rust, etc. You can get chemicals to remove paint at auto parts stores. If that doesnt work sand blast it or hand sand it. Get a good quality primer too.
I've used Fusion on a couple of bikes and it seems to work well. I also used it on the air intake vents on my boat that were severely yellowed from UV. It's held up very well on the vents for 3 years now exposed to direct sun all summer long. The only negative thing I can say about it is it doesn't cover very well so it will take multiple coats. Everything I've painted was basically the same color as the original plastic.
Fusion will look great on the plastics, for a little while, but don't expect it to hold up very well. Yes, I have done it and now the bike looks like crap; I will be stripping the stuff back off whenever I get around to it.
painting plastic is a bad idea. but if you must i would reccommend going to your local auto paint supplier and ask them if they have something for plastic. as for the metal use a good quality auto paint. and yes you will have to buy a spray gun, but the results will be so much better.
please please put the spray bombs away!