Final answer:
The relative color of blood indicates the amount of oxygen bound to hemoglobin.
Step-by-step explanation:
The relative color (depth of redness) of blood is a direct indication of the amount of oxygen bound to hemoglobin.
In the lungs, hemoglobin picks up oxygen and forms oxyhemoglobin, which is bright red in color.
As the oxyhemoglobin releases some oxygen molecules to the body tissues, it becomes deoxyhemoglobin, which is darker red.
The oxygen binding and releasing process determines the color of blood. Carbon dioxide, on the other hand, forms carbaminohemoglobin when it binds to amino acids in hemoglobin, but it does not affect the color of blood as significantly as oxygen.