The problem is that the premix gas vaporates right through the plastic in time. Therefore, the miscoloration is not only on the surface, and cannot me polished, or sanded off. All attempts to paint the tank will fail. You will, sooner or later be able to peel off the paint making everything look worse than ever.
Furthermore, the decals won't stick.
The only permanent and lasting method is to make a tank skin. There are skins available from various sources. It is less work and cheaper to buy one of those instead of making your own. But if you can't find a skin or would like to make one yourself. This is "one" way of doing it.
Step 1: Method and material
The skin is made from fibre glass vowen, epoxy, putty and paint. It is possible that polyester based resin can be used instead of epoxy but I do not recommend it. Cured polyester will let vapors through and the risk of ending up with a skin with the same problem as the tank isn't appealing.
You need vowen for epoxy and real epoxy. Marine grade epoxy such as System three or MAS or others will do the job.
The putty can be made from mixing a filler into the epoxy such as small baloons or wood flour. Readily available 2 component putty can be used instead of course.
I recommend polyurethane 2 component paint. It will give a UV resistent, nice and durable finish. Any paint shop or marine paint shop will be able to provide you with a decent paint.
In short, the method is to saturate fibre glass fabric with wet epoxy and to build up a skin. 3 layers of vowen is sufficient to have a strong and rigid skin. Thereafter, you will apply putty, sand, more putty until the surface is decent enough the paint it.
Finally, a couple of layers of pant with some sanding will give you a nice finish and a lasting result.
Step 2: Apply vowen and epoxy
Mix resin and hardener with the ratio according to the bottle. it is very important to mix it carefully in order to have it cure as it should and reach maximum strength. To much hardener can have the resin to cure to fast making it to weak. To little hardener can leave it uncured forever. Never mix to much at the time.
1. First, apply resin onto the tank using a paint brush. That way, it will be easier to apply the fabric. Lay out the fabric. You can work with smaller pieces or half the tank, doesn't really matter. Make sure that you saturate the fibre glass vowen throughoutly squeezing it lightly with the brush. It is supposed to become almost invisible. Let it cure for 8 - 24 hours.
Then build up another layer and finally the third using the same technique.
Step 3: Putty and sanding.
Then It's just adding putty..sanding..putty..sanding until the surface feel smooth and fair and look ok for painting.
It is way easier to spend 1 hour more doing this and save 3 hours sanding and polishing the paint afterwards.
Step 4: Painting and finishing
I paint one side at the time. Trying to build up a fairly thick layer. Then I sand it to smoothen it out once before the final coat. I will sand that as well using, first a 600 grit, then a 1200 grit paper. Finally, you can using a car polish to make it shine or stick with the 1200 grit finish which is quite close to the original plastic in finish.
BTW, I have protected the cap threads with some tape before I even started doing anything.