Just about all dogs have either pink tongues or at least some pink on their tongues. If that pink color appears to “fade” somewhat, take your dog to the veterinarian. It could be a sign of anemia. If the tongue literally goes gray or bluish-gray, he could be suffering from oxygen deprivation as a result of heart disease or a lung problem.
Gum color provides clues to your dog’s health, too. The gums (unless your dog has black gums) should be pink. If they are blue, he lacks oxygen. If they are white, he has lost blood, perhaps via internal bleeding.
Try doing a capillary refill test on your dog. Press on his gums with a finger. When you remove the finger, the gums should be white but return to their normal pink color within, literally, a couple of seconds. If they don’t, something’s wrong with his blood flow. There may be too little blood, or too little oxygenated blood.
You can’t do a capillary refill test on a dog with black gums, but you can check for pink tissue on the inside of the lower eyelid by gently pulling the lid down.