I asked just this sort of question at Tandy Leather.
The problem is that a leather holster is probably dyed, then waxed or lacquered, then polished. The new shoe polish will not likely penetrate the watertight finish of the holster. Furthermore, the polish is a combination of black and shiny, not a pure dye.
Your shoes may have worked because the wax or lacquer finish had worn off to a large extent.
If you are going to dye finished leather, there are strippers you can use (get `em at Tandy), then liquid dyes, then liquid waxes to finish (and wax over the dyed leather helps reduce color transfer to your shirt and pants).
Hope this helps. And I hope that I eventually get around to finishing my first homemade leather holster.
