Blood is ALWAYS red. Oxygen-rich blood is bright red as it leaves the heart. When it returns in veins without much oxygen, it’s still red, but it’s a deeper, darker red. But our veins look blue. Ever wonder why?
The colors we see are the result of which wavelengths of light are reflected back to our eyes. Veins appear blue because blue light is reflected back to our eyes even though veins contain deep, dark red blood. Blue light does not penetrate human tissue as deeply as red light does. As a result, veins that are close to the surface of the skin will be more likely to reflect blue light back to the eye. The oxygen-depleted blood that veins carry also tends to absorb red light more than blue light. When the dark red blood absorbs more red light, blue light is more likely to be reflected back to our eyes.