I like both the ivory and the walnut - if I had to choose though, I'd go with the ivory!
In regards to darkening up the ivory - I really don't have any idea if this would work or not as I don't have any experience with "faux" or synthetic ivory - but - for "aging" powder horns, I use a product called "Old Bones". I don't have a bottle of it handy so can't tell you for sure, but I think it is supposed to be diluted in water to soak the horn in to age it. I've always just applied it straight from the bottle with excellent results. Of course this is on cow horn - as I say, I don't have any idea if or how it would work on "faux ivory". Has anybody ever tried it? If sombody does try it, I'd recommend doing it on the inside of the grip first to see if or what the results would be. Just a thought . . . . . .
"Old Bones" comes in a small bottle and I'm pretty sure it is available from Track Of The Wolf and probably other dealers as well. As I said, test it on the inside first as I have no idea if it would work or what the resulting color or shade would be. If someone has tried it, I hope you'll post and let us know if it works.