I've been quite a advocate of Vallejo since pickup them up many years ago, at first principally for their solvent-free formula, but quickly discovered they brush painted much better than any other acrylic I tried....when thinned of course (out of the bottle they are almost gel like).  For both brush and airbrush I would thin them with Vallejo's own thinner medium.  Since I tend to airbrush more than paint brush, I started buying ModelAir and although pre-thinned for airbrushing, they brush paint very well as well (think about it: you have to thin ModelColor with something, anything, anyway).  Might be a touch thinner than you are used to for brush painting but it skins over so freaking fast that you can do a thin coat on, say, a 1/35 figure and by the time you are done the place you started will be dry enough to go over again so you can get 2-3 or however many THIN coats you want in one setting.

Getting back to the OP for a second, yes different pigments will have different coverage.  Several thin coats is good advice for any paint, even ones you can otherwise get opaque in one pass, but like I said it doesnt mean you have to do it in several sittings.  Even particularly "weak" colors like the cadmium-free yellows of the ModelAir (which require a white base even) can be given several fine coats in one sitting (if what I'm painting is really small I just hit it with air from the airbrush till it skins over then do another pass).