You're fine. Poop is naturally brown due to the bile produced in your liver.
Food may be moving through your intestine too quickly. Or you could have eat alot of green leafy veggies, or green food coloring.
Greasy, foul-smelling yellow popp indicates excess fat, which could be due to malabsorption disorder like celiac disease.
It could mean that you're bleeding internally due to ulcer or cancer. Some vitamins containing iron or bismuth subsalicylate could cause black poop too. Pay attention if it's sticky, and see a doc if you're worried.
Light-colored, white or clay-colored:
If it's not what you're normally seeing, it could mean that bile duct obstruction. Some meds could cause this too. See a doc.
Blood-stained or Red:
Blood in your poop could be a symptom of cancer. Always see a doc right away if you find blood in your stool.