This is a great article to refer to:
The Four C’s of Poop
Just for fun, consider this odd comparison — a veterinarian and a jeweler. A jeweler evaluates the quality of a stone based on size and color and abnormalities, using a specific set of criteria. Veterinarians do something similar, albeit on a much higher level, with your pet’s excrement. Sadly for those with weak stomachs, we often use food analogies to describe the different elements of pet stool. Sorry about that.
Color: A happy, healthy body produces chocolate brown stool. This color comes from bile, a fluid released from the gallbladder to help digest food, and bilirubin, a pigment in the bile. Though some variation in color is normal, certain color changes always catch our attention. Bright red streaks may indicate bleeding low in the GI tract, while tarry black or maroon stools can be caused by bleeding in the stomach or small intestines. Clay-colored or pale yellow stools can be caused by problems with the liver, gallbladder or pancreas. Any significant deviation from the chocolate brown color that persists for more than one or two stools is cause for concern. An exception would be color changes that reflect the pet’s diet, such as kibble containing food coloring that results in flecks of color.
Consistency: You may be surprised to know that some veterinarians use a numerical system to score the consistency of a pet’s stool. Yes, we get that specific. The fecal scoring system assigns a value to the stool from 1 to 7, where 1 represents very hard pellets and 7 is a puddle. The ideal stool is a 2: a firm segmented piece, caterpillar shaped, that feels like Play-Doh when pressed. Some pets naturally have squishier poops than others do, but all stool should hold its form. A single cow pie is rarely a cause for concern, aside from the question of how to pick it up off the grass, but call us for formless stool lasting more than a day.
Coating: Stool shouldn’t have any coating; you should be able to pick up the perfect poop without leaving any residue on the ground. A coating of mucous often accompanies disorders of the colon. Bright red blood may also be present in a pet's stool, which is always alarming for owners. Though a single streak of red on a stool can happen for a variety of reasons and is often not a cause for concern, bleeding that persists for more than one stool raises a red flag.