Yellow is a "tough" color and even the best painters can have a bit of trouble with it
. One of the tricks I have learned / used is to paint the yellow area first with another base color like a buff or light tan, you can also then do a bit of high lighting or shading to the base color. Then paint on your yellow it will look great! Yellow paint EVEN good yellow is more opaque the solid color when compare to most other colors, that's just the way it is. 
Yep, always paint a layer of yellow with a lot of white mixed in first (or something light and opaque, or tan as Baconfat suggests),
then the yellow you really want.
Yellow is a bitch in any media really, not just acrylic on minis

I don't know how all the people that prime black do it
