I, too, have noticed that the Tru-Oil finish will amber over time, especially on American Holly. I made myself a pair just over 6 months ago and they are already starting to amber. I can't wait until they are as old as yours.
Question for you folks: Do you find the lighter colored grips get discolored from hand oils? The even aging patina is one thing; It's natural and wonderful looking. I'm talking about dirt showing up as uneven splotches.
I'm considering getting a set of quilted maple grips for a revolver that I sometimes carry. But if they're going to end up grungy looking in a year I'll get a darker wood.
My personal set of American Holly grips cleaned up nicely with a wetted rag after the hunting season was over. The oil finish (especially mine) is rather thick and smooth and is not so much of a dirt magnet.
I have quite a bit of American Holly wood for sale if anyone is interested. The wood is not dry but cut into slabs before shipping. The tree came out of Southern Mississippi and was about 18" through from side to side.
I have been drying my small grip size pieces in a Microwave with good results. Pricing upon request.
You had better dry that wood quickly. American Holly will grow a green fungus/mold/something that will stain the wood. I've received some material where it was present, but my current vendor has the process down pat.