Why the carbon dioxide can change the blood from red to dull red?

Carbon dioxide (CO2) affects the color of blood due to its interaction with hemoglobin. When oxygen binds to hemoglobin in the lungs, it forms oxyhemoglobin, which appears bright red. As oxygen is released to tissues and CO2 is taken up, hemoglobin becomes deoxygenated, leading to the formation of deoxyhemoglobin, which has a darker, dull red color. This color change is a direct result of the different forms of hemoglobin and their binding states with oxygen and carbon dioxide.