This is not walnut, but white oak retouched to resemble walnut, wood grain is still very oakish.
I guess that from there, it is much easier to imagine slightly more red but similarly dark rosewood. Same goes for other hues. I can do it too, although colour appearance on the web is slightly different than in imaging software, plus oak grain looks awkward in some colours.
If I remember correctly, ekovalsky owns walnut RM40 and pictures were available on the web.