Agree with most of whats above. Tube paints can be used fine though but like airbrush paints if you want quality it costs. You can get acrylic tube paints that have high quality and finely ground pigments, but again these are at the top of the market in cost. They do reduce well and if you use a flow medium for a lot of the reduction (Instead of 100% water), strain as mentioned, pigment binder breakdown can be kept at a minimum. You can also add glycerin to the mix to reduce tip dry and some also add windex. (Window cleaner), though personally I don't like mixing chemicals like windex into my paint so I use a flow medium like Jo Sonja's. If you want to use cheap paint acrylic tubes, lots of the issues stated above are evident as cheap material and pigment has been used in their creation.
When you way up the costs of buying expensive tube paint and mixing it up yourself you may find yourself slightly ahead of the game as one tube will mix to quite a fair bit of paint pending on tube size but it does cost time and you may find drying times and such affected and is really a trial and error thing to find the right mix...Using the cheap tubes though is great for early practice, but when doing serious work it sometimes pays to get a product especially designed for airbrushing as a lot of guess work and ruined pictures will be removed from the equation..Personally I don't use many of the common brands as I find them extremely expensive for such small amounts. Theres a supplier who makes specialised inks for airbrushing in Australia and I'll happily give em a plug as for $150 you will get 5 x 500 ml bottles which will last close to two years of pretty solid painting..
PS-Buy A transparent primary set and an extra bottle or two of white and no color besides your metallics and such can't be made.....
Heres a link-
Airbrush Focus (1300 AIRBRUSH)
GL and av fun...
