I'd highly recommend getting an airbrush for both priming and base coating. Vallejo Polyurethane primers are good and cheap compared to rattle cans, less susceptible to atmospheric conditions and can still be brushed on. Come in white, (light) grey and black, along with a bunch of colours. Also great for basecoating!
This BuyPainted video shows how to paiont and shade yellow (specificially for imperial fists) nicely and easily with an airbrush.
Definitely better to do yellows over white or grey especially if you're brushing it on.
Specificially re. Orks, I recall seeing a great tutorial here on Dakka with an Amry Painter yellow primed Ork, with green washes used on the skin. The yellow basecoat combined with green washes results in a nautral light green highlight on the surface of the skin whilst you get nice dark green shadows. Doubly awesome because it does both the skin like that, and leaves you with a nice yellow base to work with for your cloth.