The element primarily responsible for the coloration of blood in humans is iron. Iron is a key component of hemoglobin, a protein in red blood cells that binds to oxygen. When oxygen is attached, hemoglobin appears bright red, while deoxygenated hemoglobin has a darker hue. This iron-containing molecule not only plays a vital role in transporting oxygen throughout the body but also contributes to the distinctive color of blood, which is essential for diagnosing various medical conditions.