I get Mason or Ball Canning Jars.. Baby food Jars or any old jar will work... put some Lacquer Thinner in the jar... Take the tip off your airbrush and back end off everything but the needle.. then soak it in the thinner for 2 or 3 mins and fill up your paint cap with it.. then take the needle and move it in and out of the gun manually with your hand and let the thinner drip out slowly or under the thinner in the jar.. watch as all the old dried up junks gets out of your gun and fills the jar...I have guns I swear were clean then I did that trick and I was amazed at how much junk was hidden......A happy gun is a clean gun.
I do that about every 2 weeks to all my guns.
Between colors try this stuff...Purple Power and Water mix ratio 10 to 1...you can buy purple power at Walmart... Works on any water base paints.
If all else fails.... SPIT IN IT! Skoal works better than Beechnut!