Sounds like a good plan for color mixing. Golden has an airbrush medium you can use to thin their paints, of course that would probably be just as expensive or more. I've had success when just messing around using craft paints. Not the bottom of the line 50 cent ones (the pigment is too rough) but the same kind of brands $1.50 ones. Ive had a lot of success thinning artist acrylic with just water and liquitex varnish and it works great until you get into tiny details. If you thin it enough to work right then on small details it just spiders all over the place. It really does no good for learning how to thin the paint you will actually be using though.