Urine is yellow primarily due to a pigment called urobilin. This pigment originates from the breakdown of old red blood cells. When red blood cells reach the end of their life cycle, a substance called bilirubin is produced in the liver and secreted into the gut. Gut bacteria convert bilirubin into a colorless substance called urobilinogen, which then degrades into urobilin, a yellow pigment that colors the urine. The enzyme bilirubin reductase, produced by gut bacteria, is responsible for this conversion process. Additionally, the shade of yellow in urine can vary based on hydration levels—more water dilutes the pigment making urine lighter, while dehydration concentrates it causing darker yellow urine. Dietary factors, vitamin intake (especially B vitamins like riboflavin), and certain medications can also influence urine color, sometimes making it appear bright yellow or other shades. In summary, the yellow color of pee comes mainly from the pigment urobilin, a byproduct of red blood cell breakdown processed by the liver and gut bacteria, with variations influenced by hydration, diet, and supplements.