Sand it, get a paint that is for plastic and try it. I had an old Honds that I painted and it lasted about three months. The paint is cheap so I just put a fresh coat on every so often. If you plan to two tone it or stripe it you may want to re think. But like Predator said dont let the negative talk hold you back. Do what you want, if it cant be worse than it already is you realy have nothing to worry about.
The trouble with those old Yamaha tanks is that the discoloation is caused by fuel permeating through the tank. They brown from the inside out so you can't really sand it off. I also highly advise against even attempting to paint the tank. It will look good for a very short time. But, between the poor adhesion between plastic and paint as well as fuel vapors attacking the backside of the paint, the finish will be very short lived.