Fresh cut walnut if full of water and darker. It gets lighter as it loses the water. Exposure to uv light will lighten it more. I have some unfinished walnut that has been exposed to sunlight for several years and it has lightened some.I have some that has been exposed for 20 years and I can't see that it is any lighter after a lot more exposure than the preceding example.I just looked at a 120+ year old walnut dining table. The top has been 'scrubbed" of any original finish and it is a sort of light brownish-gray in color. The legs,with their original finish, are a nice reddish color. This table has been exposed to light for the last 75 years that I know of.Get under the surface wood a few thousanths and you will lose the appearance of age and sun exposure.I have thousands of feet of walnut and dozens of pieces of walnut furniture to examine at every stage of finish and exposure and do so frequently in order to get the "right" look.Fascinating.
