If you toss a bright red toy into the green grass, your dog can find it if the toy is bouncing or rolling but might not if the toy has stopped moving. The toy’s location appears obvious to us, but not to our dogs.
Dogs were once thought to be colorblind, but more recent evidence suggests that dogs see a limited range of colors, much like a person who is red-green colorblind. Dogs probably see in a range of yellows, blues, violets, and grays.
For more details on the vision of dogs, go here.