Yeah, I have the natural NST boot from Epaulet and they have darkened over the year I've had them. Montana Pitch Blend darkened them as well. I'd go that route and take it slowly. I think that brown is probably hard to recreate. Check out pictures of aged natural on-line, it develops great color on it's own. Or maybe try to test the polish on the tongue or something if you feel you want to try it.