A pet may rub or scratch their eye when inflammation is present. If painful, animals may also squint. Rods are for sensing motion and work best in low light levels. All mammals, including people, have more rods than cones. Cone cells are adapted for seeing in brighter light and can detect different colors of light.
Humans have three types of cones.
Dogs have two types of cones. Evidence suggests that the dog has vision similar to a human who is red-green color blind. Cats have three types of cones, like people, but do not have exactly the same color vision as we do. Cats live in a world of fuzzy pastels. Dogs and cats appear to respond to blue and yellow best, and seem to have more trouble with green and red.