For enamels you can use lacquer thinner. That stuff will
clean anything. Don't smoke while you are doing it though. LOL!
Seriously, it is very hazardous. Do it in a ventilated area and
or use a mask.
For acrylics, water based, water should work.
For chemical based acrylics, a thinner should work.
Also, if you use the same colours all the time, you can just
keep the left over paint in the airbrush jars. I just ordered twelve
3/4 oz and six 2 oz badger glass jars and that's what I will
be doing.
BTW, if you keep enamels, thinned with lacquer thinner, in
a jar, make sure it is glass. The thinner will disolve the plastic
jars.
Maybe others have additional suggestions as well.
James