Ah, ok - if the title and description stated silver/grey and a different colour was sent, then I would personally either offer refund on return (including return postage costs), or exchange if I had the silver/grey (again, covering all postage costs).

If they were prepared to keep the phone, I would offer a reasonable partial refund (as it would save at least one lot of postage). 

You can put the title of the listing here for members to search and check for further advice. AFAIK, a direct link is not technically against board posting rules in these circumstances, but they are sometimes edited out anyway.