If our blood is red, why are our veins blue?

No matter what people try to tell you, our blood is always red. So why do our veins look blue?  
Well, first of all, you need to know that blue light and red light have different wavelengths, and consequently are absorbed and reflected differently. Red light (620-750 nm) penetrates the skin more readily than blue light (450-495 nm) does. Veins look blue because although the blood is still red, blue light is also reflected. The veins which appear blue are actually deeper in the skin, and blue light (which would normally be absorbed) has a hard time penetrating the tissue, so more blue light is reflected back than normal. What's interesting is that more red light is reflected back than blue light, yet the veins still appear blue. This is where your skin comes in. Skin itself, not the melanin pigment, doesn't absorb much light, meaning it reflects light of all wavelengths. Even though there's more red light than blue light being reflected from your veins, your brain corrects for the red light being reflected by your skin, making your veins appear blue relative to your skin.