We are having the same argument in my house.
I believe that it is more that they are receptive only to certain colours,and don't get the full range we do.It is something to do with the rods and cones in the eye.
Lankeela or Divegirl might have a better idea.
dogs colour vision is not as accurate as humans, though they can differentiate between blues and yellows they are unable to do the same between red and green - this is as said due to the number of rods and cones in they're eyes (these are the colour receptive elements)...
so no dogs aren't colour blind though they are colour limited!?