Final answer:
The color of urine linked to the breakdown of old red blood cells is mostly due to the concentration of urobilin, which is a by-product of the metabolism of heme.
Step-by-step explanation:
The color of urine associated with the breakdown of old red blood cells is primarily due to the concentration of urobilin (or urochrome), which is a breakdown product of heme from hemoglobin. After red blood cells are broken down, their heme is converted into biliverdin and subsequently into bilirubin. Bilirubin, transported to the liver, is processed into water-soluble forms for excretion. A portion of this bilirubin is transformed by intestinal bacteria into urobilinogen and further into urobilin, which imparts the characteristic yellow color to urine.