I've seen lots of GSDs with variations on the normal sable type colouring, and I've seen black ones and white ones, but at our grooming college this week there was what I can only describe as a black and tan. He was black all over except he had the bottom half or 2/3 of his legs tan. Gorgeous! Is this a recognised colour or another one that's unrecognised like the blacks?

Hi Lucy what you saw was what is known as a "bi-colour".  This is a recognised colour in GSDs and is quite common.  All blacks are recognised too, it is whites that are not recognised as paling is deemed to be a fault and no reputable breeder would knowingly breed these.  Obviously you do get people deliberately breeding whites as there is demand from pet people for them and some are nice, but you have to consider the restricted gene pool in use and wonder if the required inbreeding will throw up other problems apart from the colour. 

Oops sorry, I can never remember if it's blacks or whites you can't show, being completely not my breed or even my group!! I see why people shouldn't breed whites, but it's a shame the colour is likely to throw other problems as I think they're beautiful! But then so are the usual colour ones. :-) Thanks John!