The neck will be dark, the top will be dark. You're already tinking walnut, so the body will be dark.
From an aesthetic viewpoint, I think that'd be really dark. No contrast or interest visually.
But then again, I'm a big fan of high contrast. My art teacher pounded it into our heads: variety creates interest. All dark = no variety.
Think about some white limba for the body. The tone is similar to mahogany and it'd create a visual difference. Maybe some cream binding around the body. If it's a dark f/b, maybe cream binding on it as well. Of course, if they're both bound, you have to do the headstock as well, just for continuity.