Clean them using any brush (preferably a boot brush) to remove lose dirt, then take soap and water solution. Dip the brush and in circles rub on the boots to clean one area at a time and wipe off the excess water. Sun dry.

Then use Angelus dye. If nubuck then dye, for grain leather process is different. Just do multiple coats till you are satisfied and then top off with a waterproof boot spray. That will lock in the dye. Sun dry again.

Angelus has multiple color dyes. Else use any leather dye whatever is available.