Your cat needs to see a vet immediately to rule out serious illness. Any yellowing of the mucous membranes or tongue can indicate elevated levels of a chemical produced by the liver. There are three possible causes of such an elevated level: An excessive breakdown of red blood cells; Primary liver disease; Bile-duct blockages. It is therefore possible that your cat has a very serious illness. However, only your vet can diagnose your cat and reveal if there are any other possible causes.