Iyanden Darksun or Tausept Ochre are the yellow base colours in the Foundation range.
Most people tend to undercoat with white but you can get a nice rich yellow with undercoating black.
I like this guy's paint job on his imperial fist army.
Take a look at this:
http://cbstudios.co.uk/#/tutorialimperial/4525393843
Keep your paints thin though, especially foundation as they are designed to be pigment heavy. They were designed to cover as a base coat in one go, but
id still water down and do a few layers like you did.
Did you paint the yellow straight onto your metal/plastic mini? If so that could be why it is unsmooth and 'dodgy'. Paint needs something to adhere to such as a undercoat/primer (paint will hold onto the surface of the undercoat better than straight plastic) , that's why you should always undercoat a model.
Also before you assemble and paint your mini's, give them a rinse in water with a dab of soap to get rid of the film or dust that the molding process can create - Paint reacts with this and can cause to go lump/discolor and nt bond as well.
If I'm telling you how to suck eggs I'm sorry.
Have a search on google or here on
dak dak for 'painting imperial fists', you'll get some great ideas.
Slappy