I have tried yellow once before, with the old paints. The problems I encountered then where, painting stripes, the yellow wasn't completely yellow and you needed alot of coats.
Today I tried the
GW yellows for the first time (the drop pod was airbrushed with vallejo yellow). It was flash git yellow, so far, I haven't encountered any problems. The yellow covered in 1, sometimes 2 coats. It had no difficulties covering the brown wash, even thinned down with water.
I have made a few lists, but can't deceide which one to build. The core is the same, so I start painting that and as I get some games under the belt, I hope I can make the decision.
So far I need:
2 tactical squads with lascannon and melta gun
1 scout squad with sniper rifles and camo cloaks
1 vindicator
devastator squad with missile launchers
ironclad dreadnought in drop pod
sternguard with 2 combi melta and drop pod
Edit: Here are some pictures of marines with a layer of
GW flash gitz yellow
Here I painted just the helmet, hope the difference is enough on photo