The Merck Veterinary Manual says redness is one of the five clinical signs of inflammation. So when a dog has a red ear, something's not right, and it's time to call your veterinarian.
The "Dog Owner's Home Veterinary Handbook" lists bacterial and fungal infections of the ear canal as common causes of ear redness. Allergies can also redden the ear.
According to the American Animal Hospital Association, your vet will examine the ear with an otoscope, which shines light into the ear and magnifies the view. If there's discharge—as is usually the case with infections—your vet will swab the ear to see what organism or organisms are at fault. Discharge isn't present with allergies unless they cause a secondary infection.
With bacterial and fungal ear infections, you have to keep the ear clean. Your vet will prescribe an antibiotic or anti-fungal ointment to squeeze into the ear. For allergies, 1 percent hydrocortisone cream on the red areas soothes itching.
Depending on how much pain the dog is in, the animal may not want you touching the infected ear during treatment. Ask your vet to show you how to do it safely. Try to keep the dog from scratching its ears, which could make the problem worse.
Keeping the ears clean on a regular basis prevents infection. If your dog has allergies and you don't know what's causing them, ask your vet about allergy testing.
Don't put off calling your vet at the first sign of redness or discharge. The longer you wait, the redder that ear may get. Extreme reddening indicates a serious infection that may require sedation to clean the ear for initial treatment.