Ok, I can see the listing - the only picture is of a silver phone, plus the title says silver and there's not anything to indicate the buyer needed to make a colour choice, so anyone would have been expecting to get a silver phone. Do you know what colour they did get? I ask as the listing states the item location is China, and I'm presuming it was sent to the buyer directly (i.e. dropshipped). 

A partial refund might be the quickest way to resolve, but definitely give the buyer the choice between that or returning for a full refund.