Well, this is an easy task. I found this to be quite a bit easier than I imagined it would be.
For my leather armor, I wanted some different colors. So, I talked to the shoe repair man who works with leather every day. He sold me this dye.
There are several brands out there. I am sure they probably all work pretty decent.
I will dye these brown leather straps black.
Now, a little note. Vegetable tanned leather is basically raw. It will take almost any color leather dye. Leather that is not vegetable tanned, is tanned with other chemicals or processes. Which means, they may already have a color to them, like my brown leather above. With chemically tanned leather, you may not be able to dye light colors. I can only dye my brown leather darker, not lighter. So, most leather goods can be dyed, but usually you can only go darker. You are not going to be able to take black boots and dye them white.
The leather dye comes with a dauber/applicator. Put some latex gloves on, and work outside. Then just brush it on. It is super easy. I dyed the top, sides and back. You can see the one middle black strap in the above photo.
Let it dry, then apply another coat. It has a smell to it, not super strong, but it is best outside or somewhere with some ventilation.
When it is dry, it may leave a haze on some leathers. Buff this off with a clean cloth. When you are done, apply some kind of finish, like shoe wax, or leather oil. I even used vaseline. The finish will help seal in the color, and make the leather last longer.
Dye the pieces separately before you attach them to the armor. It may bleed if you dye the pieces in place. If you absolutely have to dye something next to something you don’t want dyed, then use a tiny art brush to carefully paint on the dye. Be super careful.
Go ahead, make a mess, have some fun.