I find vinegaroon will turn leather more of a gray color as it looses it's strenght, if gray is what you're looking for a weak solution of vinegaroon will produce a gray,(old vinegaroon) or if you're wanting to gray a walnut dye you can just add that weak solution to your walnut dye, go easy, too much iron will darken fast, but experiment with it until you get the color you're after, and there again GREEN BLACK WALNUTS will give you the strongest deeper color, and on another note, adding more vinegar and iron to your vinegaroon will bring it back to full strength, as a rule I like keeping several large tub containers with different strength levels of both vinegaroon and walnut dye, that's just me, with my new shop I don't have the room I use to for storage...so I'm for a new way to store dye...