Dogs can't see colors. One thing is for sure: people can't see what dogs see, so it's not clear where this idea came from
Dogs Can See Different Colors—Not True A expert for doglab.com, veterinarian Sara Ochoa, DVM, says, "Dogs just don't see as vivid colors as we do."
It is said that dogs see mostly blue and yellow. This is because two of the three types of photoreceptors needed to see color are in the retina of dogs.
When dogs are sick, they eat grass. It's true that some dogs eat grass when they feel sick.
It's not true that dogs eat grass when they're sick. People who work at The Puppy Academy say, "Dogs will often eat anything!"
Seeing a dog's tail wag means it's friendly. A dog's moving tail is often the best way to tell if it's happy.
If you want to be sure you're reading a dog correctly before thinking it is friendly, the Puppy Academy says to look for other behavioral cues, like eyes and ears that are relaxed.