Just changed my mind about using a water based topcoat on walnut after reading a study from the University of Tennessee's wood science program | Furniture Finishers - Opinions Matter
Wait, that study is about water based finishes in general, not topcoats. The yellowing they talk about happens with water based stains and dyes because they can react with tannins. A clear water based topcoat on walnut is actually a solid choice if you want to keep the natural color from changing over time.