Osage and African Blackwood to name a few woods are all effected by ultra violet light which over time will cause the wood to patina and change color. I too love to see how the different woods age over time. I have an African Blackwood call that when I turned that call for myself was one of the most beautiful pieces of AFB that I made a call from with beautiful visible grain. Now that same call is as black as a piece of coal. The grain is still there but I almost have to put a pair of magnifiers on to see it that's who much it has changed color.
To slow down the process keep the call in a dark place with the least amount of exposure to UV light, that at least will slow down the process and has been my experience.
But as a call maker, and I am sure I speak for pretty many of us we like to see our calls being put to use so that pretty much blows that theory out of the water, LOL.
