I have found Air brushing to be pretty much the same on any materials, leather or metal just different paints or dyes depending what they are being sprayed on. Air sick has some good online tutorials on basic airbrushing, from basic operation to doing skulls and such. Doing things like basic color fading or darkening on leather would be done with dyes. You just thin them to get the desired color. How easy it is to do would depend on the air brush. Smaller needles would allow finer detail work but they will also clog easier. I have a harbor freight special that I use for general dying and finish work it and it works fine for me. You just have to play with it to figure out what pressures work best for different things. There are some much more talented people here that do very fine detail work that can give better answers on this than me. Hope this helps some.
David