Based on the fact that Walnut turns orange as it ages and directly proportionately to sun exposure, I stain everything with black or dark walnut stain prior to doing anything to it, regardless of the topcoat choice. Most clients have no idea of this effect until a few years in the sun does the deed. Watching a natural deep brown, classic walnut kitchen go pumpkin orange over the years is really no fun. Of course, topcoats used claim UV resistant properties, but mother nature always wins in the long run.
A pigmented stain helps protect the color a little as well as deepens it in hopes of reducing the garishness of the amber/orange patina that is inevitable.
If I owned a walnut instrument, I'd only take it out under the light of the stage. In fact, I have a rosewood instrument that suffered the same fate....
I recently revived a 20 year old Chapman stick made of rosewood that had gone almost a banana yellow, based on previous owner's lack of maintenance and likely leaving it out on display in a sunlit room for years. It was outright shocking how much deep ruddy brown was hiding under that oxidation, but it took a judicious amount of sanding/scraping to reveal. Of course, I used a dark walnut colored Watco to restore and seal, given the fact that once again, it's a beautiful dark wood that goes straight to hell with too much sun exposure.
Beautiful instruments, all the above, just be aware of this reality of color shift over time. It happens gradually enough that you may not notice immediately, but a photo from a handful of years before and after sunning will make the effect obvious.
FYI, FWIW.
