Normal stool is usually brown. Green stools can come from leafy vegetables or fast transit; very dark stools from iron supplements or some foods; and reddish colour from beetroot or food dyes. These dietary causes are harmless and settle once the food clears.
Certain colours do matter. Black, tarry, sticky stools can mean bleeding in the upper gut and should be assessed urgently. Bright red blood may come from piles or lower gut bleeding and should be checked. Pale, clay-coloured stools, especially with yellowing of the skin or eyes and dark urine, can indicate a liver or bile duct problem and need prompt medical review.
If you cannot link a colour change to food or supplements, or it persists, see a doctor. Seek urgent care or call your local emergency number for black tarry or bloody stools combined with abdominal pain, vomiting blood, dizziness or fainting.