Just gave Revel (Germany) Aqua color a spin. I was very impressed. It's a water based acrylic that is very vallejo like. Revel claims that Aqua Colors are "self-leveling" and it was true that in the area I painted by hand it's very hard to detect a brush stroke - and I'm not much of a painter. They recommend water for a a thinner - but not more than 25%. I use flow aid and retarder - available at any artist supply store and good stuff to use with any water based acrylic. You have to order the stuff from the UK where it's widely available but the prices are low and stuff like that ships quickly and cheaply. There are 88 colors, heavily weighted toward military, especially German, hues. This is the most benign paint I've ever used except for artist acrylics: you can't smell the stuff and it cleans easily. (It doesn't rebel if you use lacquer thinner to clean the airbrush thankfully: nothing works better.)
Just stumbled on AK Interactive's US website. http://www.ak-interactive-usa.com/index.html. To the best of my knowledge nobody in the US carries AKI's stuff - enamel based washes, filters and pigments are doing very well in the UK. (Mig Jimenez is their guru after his falling out with MIG.) Anyway, prices are good and shipping, at least once, was extremely fast.
Eric