Hi John. Your tsuba looks a lot different to the picture in the catalogue, although definitely the same piece
it appears dark grey, almost black in there picture. Would be better to see in hand.
I think the red-brown colour comes from the more volatile elements having oxidised leaving behind a copper rich surface. For your interest please look up. (in tosogu what shibuichi looked like when it was new 01sep 2015). Please let me know what you think, we are both here to learn.
Best regards
Peter.