Mike soaks his leather in water, then sloshes the dye over the surface. The results speak for themselves
I dip dye some things; it is a very thirsty way to dye things (the leather absorbs loads), you need a sizable amount of dye to get the depth in the bath and there was another issue but I've gone blank on what it was
Probably that the extra solvent removes even more oils from the leather, so I feed it once it is dry to put som esuppleness back in.
Airbrushing gives a nice even coat - got to be a bit careful as the solvent likes to evaporate en route to the leather; I usually thin the dye down a bit for that. You can get some nice transitions between colours like that - usual caveats on airborne solvents!