Dye

I don't know if it is actually named or not beyond "stain"--in the very most basic and simplest form as a very old method of coloring leather where a powdered pigment or a dye is rubbed into the leather. It might be mixed with binder to help adhere it, like a soft wax or fat or something similar (even spit was used as a binder in some instances), but what's happening is the pigment is being rubbed into the pores of the leather.