I find myself painting a lot of yellow these days, and a method i've found that works really well is to use citadels yellow base paint to start. It used to be called iyanden darksun, not sure what the new name is. Their "base" line has much more pigment in it than regular paint, so it requires less coats to cover. On top of that, i do a very heavy drybrush (as in i only wipe off like half the paint instead of most of it, but still "dust" it on like i would with a regular drybrush) of whatever shade of yellow i want it to look. After that, i usually shade with citadel ogryn flesh.
You can see some of the results of this technique here: http://www.figurerealm.com/viewcustomfigure.php?FID=37518http://www.figurerealm.com/viewcustomfigure.php?FID=45549