Check for cyanosis or icterus (or in very rare instances a brown or cherry red discolouration). Remember that these colours are mixed in with whatever the background colour is (determined by Hb, perfusion and membrane pigmentation). A euvolaemic dog with a low PCV will have less red in the background. A euvolaemic dog with septic bile peritonitis will have an injected colour (more red) AND the yellow of hyperbilrubinaemia so its membranes will look quite a rich orange. The Shelty here has bile peritonitis and you can see he has injected MMs and with the eye of faith you can see mild icterus (he did in real life, honest!)Some cats with icterus present because their previously blue eyes have turned green!Some people say that they can appreciate a brown colour in some cats with methaemoglobinaemia from paracetamol poisoning. Cherry red mucous membranes can occur with carbon monoxide poisoning but this is very rare, even in dogs suffering from smoke inhalation.