First off, I recommend buying the color you want, dying can be risky. But, if I was going to dye them I would use Fiebings dye, you need to dilute it with denatured or isopropyl alcohol. Scrub the boots with deglazed first, then wipe them with dye, applied with Sheep wool. After the dye dries wipe the boots with a layer of néet foot oil. Then a layer of your favorite shoe cream.
You may not be happy with the results, just be ready for that.
Aaron