You don't really need dyes actually. You could do it with acrylic paints watered down. I stained my boots with heavily watered down black paint to do the grey work and a thicker mix for the middle stripe. While the boots are still wet, you can do subtle weathering with the mixtures and it will dry looking like it was dyed.
I'm a noob though, so the veterans may have other methods. I've done this kind of thing on clothing for Sideshow PF's and other costumes, which is where I learned the technique. Worked on my MOW boots though.