You may already know that arteries carry oxygen-rich blood, and veins carry oxygen-depleted blood. Many people believe that oxygen-depleted blood is blue. Blood is not blue, and it is always red in color. However, oxygen-depleted blood has a darker red color. This can be quite difficult for you to understand. Blue light can't go deep into the human tissues as red light does. It is because of the change of the wavelengths. That is why veins reflect blue light. In other words, the oxygen-depleted blood that runs through your veins absorbs red light more than blue light.