Dogs and cats have a far higher ratio of rods to cones than humans, thus their ability to see colors is far more limited. Rods are light receptors, which is why dogs and cats can see in the dark so much better than humans. Cones provide eyes color sensitivity. So basically, dogs and cats can see better at night, but at the sacrifice of not being able to judge color as well as humans (who can't see in the dark very well comparitively). Dogs also have a wider range of vision, largely due to the positioning of their eyes on their head.
well, what i've heard is that they can only see color in the dark. i'm not so sure what i think tho. i do think that like our dog always knows our car and has a cow when she sees us, although as my neighbor mentioned when our dog is at her house and sees my mom coming home from work, she watches all white cars. so, i really don't no. but, the dumbest thing i've heard is that dogs don't have memory. my psychology teacher tried to explain that to us but i never understood it. that's why i don't like psychology, its too confusing!