There is a lot of uncertainty in the world, but surely we can rely on our senses to accurately tell us about our own body, right? Not a day goes by that my veins don't look blue, but they actually aren't. Why?
The body contains arteries and veins. Arteries are responsible for transporting oxygenated blood from the heart to other organs. Once the oxygen is delivered, the deoxygenated blood travels back to the lungs and heart through veins. Because our veins look blue, a popular myth is that deoxygenated blood is blue.
While the argument sounds like it makes sense, a closer look reveals that this is not the case. Blood actually gets its red color from hemoglobin, the protein that carries oxygen through the body. With oxygen bound to the hemoglobin, the blood has a vibrant red hue. Once the oxygen is removed from the blood, it becomes a dark red color. Basically, hemoglobin is what makes blood red, oxygen only changes the shade of red.
So if dark red blood is traveling through veins, then why do they look blue to us?
In the body there is a layer of fat - called subcutaneous fat - which sits below the skin and above the veins. This means any outside light (e.g. sunlight) that wants to reach the veins has to pass through this layer. Keeping in mind that white light is a combination of every visible wavelength (i.e. every possible color), as light passes through the skin the subcutaneous fat layer absorbs low-energy wavelengths of light (reds, oranges, etc.) while allowing high-energy wavelengths to pass right through (blues, purples). This means only the blue-biased light makes it to the veins. This light bounces off the veins and reflects back into your eyes, tricking your brain into thinking they must be blue.
Other things that can alter a vein's appearance are skin color and depth of the vein being looked at.