Hi, John -
I think yellow is about the most difficult color to work with, orange right behind. First, I believe most of the experienced modelers here will agree, paint the lighter color first, then apply the darker colors.
Yellow works better when applied over white, or even a very light gray. And a primer I think is always a good idea. So, if you're going to be spraying yellow, after you have the area masked you could spray a light coat of white, then follow with the yellow or orange.
Patrick