Dogs aren’t completely color blind but they don’t see color very well. There are two types of retina cells, rods and cones. Rods detect light. Cones detect color. Dogs have many more rods than cones.
Dogs do see better than humans in the dark and they are very sensitive to movement.
Photo from http://www.flickr.com/photos/dshef/ / CC BY-NC-ND 2.0
Information from Dog Behavior: An Owner’s Guide to a Happy Healthy Pet.