You need a sealer. Other colors may mask the bleed, but when you shoot white it will look pink.
Zinsser B-I-N primer/sealer (shellac based, thinned w/91% rubbing alcohol or denatured alcohol) works awesome as a sealer; you apply it via airbrush between coats of primer and color. Home Depot/Lowes and places like that carry it, get a pint/quart can of it as the spray can isn't a good solution for spraying it. It goes on pure white, dries fast, and sands beautifully. You MUST use a light coat of lacquer primer first as the Zinsser won't "bite" into the plastic.
The Porsche below was red '70's Monogram plastic and was
very pink after several coats of Tamiya white primer. It's pink no longer. B-I-N was used between coats of primer and the Tamiya Pure White paint. Some people say that airbrushed Future will also work as a sealer. Haven't tried that.
