Look at the inside of your wrist and you will probably see a few bluish lines running under the skin. It is tempting to think that is blue blood, waiting for a lungful of oxygen to turn it red. That idea is wrong on both counts. Your blood is never blue, and your veins are not really blue either. So why do veins look blue when nothing about them actually is? The answer is not in your body at all. It is in the light bouncing off it.

Your blood is red the whole way around

Start by killing the myth. Human blood is always red, never blue (West Texas A&M University). The color comes from hemoglobin, the iron-rich molecule packed into your red blood cells. When hemoglobin is loaded with oxygen, it reflects a bright cherry red. When it has dropped that oxygen off in your tissues, it turns a darker, duller red, closer to burgundy (BBC Science Focus). That darker blood is what your veins carry back toward the heart and lungs.

It is still red, though, not blue. There is no point in the loop where blood turns blue. The proof is mundane: when a nurse draws blood from a vein, what fills the vial is dark red, every time (Live Science). If venous blood were blue, the tube would be blue. The "blood turns blue without oxygen" story is one of the stickiest science myths going, and it does not survive a single blood test.

What the skin does to light

So the blueness is not in the blood. It is in the trip light takes to reach the vein and come back. White light is a mix of every color, and skin does not treat those colors equally.

Red light has a long wavelength, so it slips through the upper layers of skin fairly easily and travels down to the vein, where the blood absorbs much of it (BBC Science Focus). Blue light has a short wavelength, and it scatters off the crowded structures in the top of your skin before it can reach the vessel, so a lot of it bounces straight back out (Live Science). Add it up over a vein and the light returning to your eye is missing much of its red and keeps more of its blue. Your brain reads that as blue.

It is the same family of physics that paints the daytime sky. Short wavelengths scatter more than long ones, which is part of why the sky is blue and why the setting sun goes red once its blue has been scattered away (West Texas A&M University). The colors you see anywhere are really a story about which wavelengths bounce back, the same logic behind why grass looks green. Your wrist is running a small version of the same trick.

Why depth and skin tone change the color

A vein right at the surface does not look blue at all. The effect needs the vessel to sit a little below the skin, where the scattering has room to work. Push the vein deeper or shallower and the perceived color shifts.

The physicist Alwin Kienle and colleagues took this apart in a 1996 paper in Applied Optics and found the color of a vessel depends on four things: how skin scatters and absorbs different wavelengths, how oxygenated the blood is, the diameter and depth of the vessel, and how your visual system reads the result (Kienle et al., Applied Optics). In their measurements, modeled vessels of different sizes and depths came out anywhere from turquoise to blue to plain red. Their sharpest point was that the perception itself does a lot of the work. A vein can look distinctly blue even when the light coming off it is barely bluer than the skin beside it, because your eye judges it against its surroundings, not in isolation.

That is also why veins look green on some people. Skin tone changes which scattered wavelengths make it back out. Under lighter skin the leftover light tends toward blue or purple; under darker, more melanin-rich skin, more of the green band returns, so the same veins read green. The blood is identical in both cases. Only the filter sitting on top of it is different.

The illusion you have been wearing your whole life

Put the pieces together and the trick is almost funny. The vein itself is nearly colorless, a thin tube of tissue, and the blood inside is red. Lay them under your skin in ordinary light and they come out looking blue, a color that exists nowhere in the actual plumbing. You have been staring at an optical illusion on the inside of your own arm since the day you were born, and most people go their whole lives sure it is the color of their blood.

Next time someone tells you blue blood turns red when it hits the air, you can tell them the truth: it was red the whole time. They were just watching the light.

Keep wondering: the same scattering of short wavelengths is most of why the sky is blue; your brain reads your own body in ways you can't override, which is part of why you can't tickle yourself; and for another everyday signal coming from inside you, here's why our stomachs growl.