Basic colors

OK, as Don mentioned, use a light primer for sure. Bomb stripes, if you can, try to paint the stripe area yellow first then use a thin strip of masking tape to mask the area you want to stay yellow, then paint the main colour.

There's no getting around it - yellow is just a bugger to deal with!

Actually it's the smaller details.  Yellow oxygen bottles and bomb pinstriping.

What do you prime the plastic with?  If it is a dark primer, maybe try a light colored primer.  I find yellow even harder to paint than white.   Even with an airbrush I find yellow usually takes several coats.  If the paint is real thin, it should not cover detail very much.  I suspect the problem is not the thinness of the paint, but the lack of opacity of those colors (especially yellow).

(freehand = not an airbrush, right)

White and especially yellow don't cover as well as darker colours, you may very well need an extra coat. What colour are you painting over? Or is this an overall colour you are painting on something? (I'm guessing you may be working on a yellow helicopter??) If it's an overall colour, you may actually want to try thinning the paint a little more, knowing that you are going to need multiple coats anyway, so you aren't covering any details. If it's just a small area (like a yellow ID band on Luftwaffe airplanes), you're better off doing that first, masking it, them painting the darker colours.

A few more details about what kind of model you are dealing with would help too. There are far more experienced paint wizards on here than I, keep an eye out for their advice also.