As I got better with the construction of my holsters, I got frustrated with the finish--Black was fine--if there is a light spot, just slap more dye on. Dark brown was OK if not spectacular, but medium colors turned out very uneven, and the clear over-coat was very hard to get smooth. I finally gave up on trying to use daubers or sponges to get a nice finish, and bought an airbrush. I already had a small air compressor with tank meant for an air stapler.
The airbrush made a huge difference, I got a mid-grade Paasche--(single action, external mix) for the first one, and it works very well--I am able to do light colors evenly, and the clear coat is miles better--Like the difference between a car painted with a roller, and one painted properly. The problem here is cost--even if you already have a compressor, few hobbyists will do enough leatherwork to justify the expense.
After I bought the Paasche, I found 3 different sets at Harbor Freight. I decided to try their 'Central Pneumatic Quick Change Air Brush Kit" on sale for $8.99, regularly $15.99
The kit contains a really flimsy looking and feeling airbrush body, made mostly of plastic with a plastic trigger, 5 thin-walled plastic bottles that snap to the body, each with an adjustable nozzle, a thin plastic air hose, and an air line adapter.
Despite the initial impression of cheapness, this airbrush kit works surprisingly well. I haven't done a whole holster yet, but I have dyed and clear-coated some scrap leather. The end result is as good as the Paasche, and absolutely worth the money. If you have a source of compressed air, it is probably worth the money even for one or two holsters.
The Paasche would likely be better for fine detail work, more controllable and a nicer trigger. The Harbor Freight's spray pattern starts at about the middle width of the Paasche's widest tip, and goes wider still--but the range appears to be perfect for most holster work. The Harbor Freight seems to need less air to run, and cleaning should be much easier with less disassembly. Since the liquid nozzles are part of the bottle, changing colors is trivial, with no cleaning necessary between solutions. I doubt the Harbor Freight kit will last as long as the Paasche, but I would be amazed if it didn't last long enough to be a great value.