The brown colour is very nice, but one though you might do is to work a little green in nearer to the "stem" to merge the two sections better.
My best suggestion is to practice on another walnut shell and see what you like best. Do one on a green that's just a little lighter, then do one where the green is strongest near the throat/stem and fades to brown, maybe some other options like a flowery red with green spots (green spots is another way you can keep it brown but add a touch of green).
Walnuts are cheap enough - might as well take advantage of that.
Once you've tried a couple of mocked-up shells, then go for the final job!
EDIT: For what it's worth, my wife says dark green with bright yellow-green highlight spots would look good.