Well yellow is a very hard colour to paint. Are you thinking you want the Yellow to be the predominant colour? or the black?
If you wanted yellow, then I would prime in black. Paint the main armour plates as yellow with black undersuit etc. Your guns etc. could similarly be black, and I would also do any plumes on the helmets black etc. If you go this way, I would go for a more muted yellow. I guess the other option would be to go with a yellow contrast paint to get the armour done. So then you'd prime in one of the contrast primers, and then just need to paint the black parts on that.
If you wanted black as the predominant colour, then what I would do is prime the whole model black, and then do a dry brush (or edge highlight, depending on what suits you) in 2 shades of yellow on the armour plates. Think of the black heart colour scheme, but switch the Kabalite Green and Sybarite Green with yellows. In this scheme, I'd have to think about what to do any of the plumes on helmets and what colour for the wyches hair. Presumably a contrast to the yellow, so something like a muted purplish black maybe?
I think I would go with option 2 myself, and I think for sure I would follow this sort of technique for your vehicles in either case.