WHY are the veins blue if the blood is red?

The bluish or greenish hue of veins is caused by how light interacts with layers of skin and subcutaneous fat. When observing our arms or legs, we perceive the reflection of light from these tissues.

While skin has limited light transmission, blood is more effective in this regard. Interestingly, red light penetrates tissues more easily than blue light. As a result, blood absorbs blue and purple light while reflecting some red light, which is why we perceive it as red. When a blood vessel is near the skin's surface, almost all of the blue light is absorbed by the blood, leading to the reflection of some red light.