I think dogs can see fewer colours than us.. I think just red and green, (but can detect motion better), but they don't need as good as vision as we do, since their sense of smell serves them the same purpose in a way. Anyway, here are a few interesting facts I found about a dog's vison:
* Dogs can see in much dimmer light than humans. This is because the central portion of a dog's retina is composed primarily of rod cells that "see" in shades of gray while human central retinas have primarily cone cells that perceive color. The rods need much less light to function than cones do.
* Dogs can detect motion better than humans can.
* Dogs can see flickering light better than humans. The only significance to this appears to be that dogs may see television as a series of moving frames rather than as a continuous scene.
* Dogs do not have the ability to focus as well on the shape of objects (their visual acuity is lower). An object a human can see clearly may appear to be blurred to a dog looking at it from the same distance. A rough estimate is that dogs have about 20/75 vision. This means that they can see at 20 feet what a normal human could see clearly at 75 feet.
-- Ohh here's an
interesting page I found about dogs vision. It actually shows the colours that they can see (definately must check this out):
http://www.uwsp.edu/psych/dog/LA/DrP4.htm