It depends on what your idea of natural color is. If you're talking brown or grey wool, then to the best of my knowledge no, but prochem does have a tutorial for bleaching cream colored wool white with peroxide. I found it a few months ago while looking for a way to bleach some ultra soft but dingy grey wool I had. In the end I just overdyed it and liked the results, so I never tried this.
It looks like alot of work, so the best thing might just be to start with white yarn and leave those sections undyed.