Thank you all for the comments.
The figures are primed with a grey auto/homestore rattle can primer. The yellows for the armor are individually mixed and layerd/blended from various citadel paints old and new. Tau sept yellow, bubonic brown, and an ancient bad moon yellow, being the three colors that I remember. I also used sepia and brown inks and washes. Not really a recipe; I figure that orks paint their armor individually and don't have much use for uniformity of color shades in their bands/units.