This pic is from this site suggesting thier specimen is the last big Rhino horn even though this is really only a small one it has to be at least 1m.
http://www.animalorphanagekenya.org/...2006_april.php
This site may explain why other colours are seen, the outer pats of a big horn may be more course and even lighter in colour?
http://www.sciencedaily.com/releases...1106144951.htm