WARNING: don't read this post if you are presently eating or have a sensitive stomach! It may prove to be gross/graphic for some.
A healthy stool (aka poop :P) color is a shade of brown. If stool is black or bright red, then there can be bleeding along the gastrointestinal tract (upper GI and lower GI respectively) due to infection or disease (please talk to your health care provider if you detect black or red stools). However, stool can turn other colors depending on what is consumed. If you eat a lot of the following products, you may see a change in stool color:
Gray stool = chocolate.
Dark green stool = grape-flavored products.
Green stool = leafy green veggies.
Black stool = iron supplements, charcoal, licorice, lead, some berries, or blood (see your doctor!).
Pink, red, or purple stool = beets, cranberries, tomatoes, red peppers, food dyes (often found in candies), hemorrhoids, or some medications.
Pale/White (chalky) stool = antacids (like Tums), malabsorption, bile production problems, gallbladder problems, liver problems (such as liver obstruction).
***ETA: Blue stool = Smurf ice cream :P
Image source here. Information source from my PCD textbook found here.