I just completed my bed out of beech. It is very hard and used for drawer sides and guides in old furniture. I stained mine with a deck stain. Mine is to be adaily driver in the north west. No clear finish will last outside so pick a finish that can be redone with minimum work. Walnut is fairly soft. The walnut will fade and turn gray over time like all wood outside. If you want the contrast look I would just stain the wood.
