I like it, but I see what you mean about the color. How about a black walnut stain brushed just in the deep parts of the border, but leave the high parts bright white? That should give it some depth and variation, especially when viewed from a distance.
Another interesting way to add color would be abalone shell. Inlay a piece of holly (or other wood) for the central carving, but border it in thin abalone, maybe leaving a micro thin strip of holly on either side of it too. Then inlay abalone for the N, and finally carve the holly around it. I'd go with the extra blue paua from Andy DePaule for the border, and either that or or green abalone heart for the N. I could send you a few pieces of either if you don't want to buy full ounces
This guitar is just too awesome!
Oh, and I like the "blocky" look of the headstock. Breaks up the monotony of the floral/curvy theme, and matches with the "frame" of the rosette. The standard Martin vine inlays are a good example of overly monotonous floral patterns, to the point that you just see it and think "yup, that's a floral pattern". I think you're doing a fine job of balancing it out