Fondation range works well too in an airbrush even with heavy dilution.
I totally agree with juicifer, you can work with a lot of different brands and pick up what you like.
Tamiya paints are great, especially with a airbrush. The only problem is that their range is covering pure military and cars, so "fantasy" colours are only available in glossy.
But if you paint imperial guards or tau for example, the range is great.
Lots of other modelling brand like Mig makes quite good stuff like very good oils already mixed with usable colours for our hobby and quality pigments. You can indeed buy some cheaper in an art store but I find it convenient already in the colour I need, and a little jar of mig pigment last years (10 tanks weathered, lots of base and troops = 1/3 of or pot) and the colour stay the same when mixed with plaster, liquids: I had bad surprises with art store pigments for this (like a nice sand colour becoming an ugly mustard one when weathering a finished tank is not a good one).
A lot of people are speaking about metallic paints, those ones have a great look:
They have a few drawbacks: you must use alcohol to dilute, and they dry very, very quickly (basicly adding 1-2 drops of alcohole every 2mins). The "The colours can be mixed with one another and dry almost instantly." is true.
BUT they look as bright & realistic as the enamel metals (humbrol etc.), without the X hour dry time. I would not recommend it for a beginner or to paint the guns of 150 imperial guards, but great for extra work on some HQ.