Yellow is the hardest color to get coverage from, IMO. If you're using enamel yellow, you're in a tough boat to row. Did you use primer? A "must" for yellow schemes! Also, with yellow you do have to use more coats than normal, esp. if you're using enamels.
I'm currently working on a SuperBee in enamel yellow, and it's a patience-tryer! One of the things you can try is to paint it over a flat yellow base; this prevents "bleed through" somewhat.
I would recommend Tamiya acrylic, painted over a base of Tamiya flat yellow. Sand the flat a bit though, to make it smoother. I have found the Tamiya yellow to be pretty good for coverage, and it dries fast too.
Their "Clear" gloss coat from the rattlecan is simply amazing!