Dogs, like most other mammals, are Dichromats. With only two kinds of cones, dogs see yellow and blue, but do not seem to be able to distinguish red and green. To them, that red dog toy blends in with the grass of your yard. If you want toys that are easy for your dog to see, stick with yellows and blues. With only two kinds of cones, it is thought that most Dichromats can distinguish about ten thousand colors.
Now, before you start to think that poor dogs are deprived of some of the world's beauty, keep in mind that some animals see more colors than we do. Most birds are Tetrachromats, with four kinds of cones: red/green/blue/ ultraviolet. So are some fish, some amphibians, some reptiles, some spiders, and many insects. Four kinds of cones could let them distinguish about one hundred million colors.
Pigeons and butterflies have it even better. They are Pentachromats, with five kinds of cones. That lowly pigeon sitting on the buildings of your city may be able to distinguish up to ten billion different colors.
Going the other way, whales, dolphins, seals, sea lions, walrus, and owl monkeys are Monochromats. With only one kind of cone, they have a very limited range of colors, and may not see color at all.