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There are two types of cells in the retina (back of the eye) that receive light: rod cells (rods) and cone cells (cones).
Rods are for sensing motion and work best in low light conditions.
Cones are adapted for vision in brighter light and can detect different colors.
All mammals, including people, have more rods than cones.
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 colorblind.
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.
What appears red to us is simply dark to the dog and cat, and green light is almost indistinguishable from white or
(a shade of gray). | 
