white paint

For years, I've painted the yellow (or white) prop tips on 1/72 a/c by marking the location of the inboard side of the paint (usually 3 scale inches), stirring up the paint, and just dip the end of the prop into the paint, just up to the line. With a little practice, it usually comes out very well. Whites, yellows and most light colors are usually really hard to paint with out brush strokes showing - although recently I have been experimenting with brush painting light colors with enamels that have been thinned with laquer thinner. Seems to leave fewer brush strokes.

Try the Vallejo acrylics, as they are the most opaque whites and yellows I have found for brush painting small details, and they dry smooth.

As Rick said, the Vallejo model color are great for brushing as they don't leave brush marks. Neat little squirt bottles too.

Priming will help with painting whites and yellows too.

If the plastic is molded in a dark or very bright color such as black, red, blue, definitely use a medium to light gray primer--that'll help the coverage when you're using light colors.