The "proper" color of the gums differs from one breed of dog to another, and may vary within a breed, too; so it's good to know your dog's baseline gum shade so you'll be cognizant when coloration changes. Atypical coloration can indicate health problems like anemia or gum disease, so you should check them routinely.
According to the American Kennel Club, your dog's gums should be pink and firm. The shade should be a vibrant bubble-gum pink as opposed to a pale pink -- pale gums can indicate health problems like anemia. A dog's gums may have black coloring or spots, which are usually normal for some breeds. If you aren't sure whether your dog's gums are normal for his breed and coloring, ask your vet to check them at his next appointment. He'll be able to tell you whether the gums are a healthy hue. From there, you can monitor them daily for changes.