Why does a dog's poop change colors?

If your dog has small white grains or "strings" in its feces, then you're most likely dealing with an internal worm problem. I suggest that you search through the possible worms that you may be struggling with on the internet, as there are several different kinds. Look for the right signs first, and then administer the proper worm medication one you know what type the dog has. Now then again, worms may not be what this is, but I can't think of any other possibilities.