by 
The Curious Bag
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Dogs are not completely colorblind, although they cannot perceive the full spectrum of light waves that most humans see. Current scientific theory proposes that dogs lack the ability to see colors on the red-green spectrum (see Reference 1).
How Color Perception Works
The retina contains cone cells and rod cells. Cones enable vision in bright light, and rods make it possible to see in dim light. Cone cells are also essential for color perception.
What Is Color Blindness?
In humans, color blindness is caused by a deficiency in the retina's cone cells that prevents perception of certain colors. Inability to perceive any color is extremely rare.
What Colors Do Dogs See?
Dogs have far fewer cone cells than do humans and more rod cells. They probably see mostly in tones of blue and yellow.
More on Dog Vision
Dogs can see much better in dim light than humans can, enabling them to hunt at dusk or at night. Dogs' eyes are also very sensitive to motion (see Reference 2).
Seeing-eye Dogs and Traffic Lights
Despite appearances, seeing-eye dogs do not "read" traffic lights. The dog's job is to watch for cars, and, if it perceives danger, to refuse to move forward (see Reference 3).
Source
Vision in Animals-What do Dogs and Cats See?, Paul E. Miller, DVM
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