This is a popular question, so I’m going to steal some from the last time I answered it.

Blood is actually red all the time, but different colors of red. When you see blood outside the body it is usually a dark red. This blood is from the veins, so it has already dropped off a lot of its oxygen to cells that needed it.

Sometimes you may cut an artery. This blood is bright red because it has not yet dropped off its oxygen. It also spurts out in pulses instead of oozing like the blood from veins. So even outside the body, blood can be different colors.

What's the difference between veins and arteries and why would that affect the color? Why does arterial blood spurt?

The blood vessels you see at the surface of your skin are veins. Everyone's skin is slightly different in color, so the veins can look different in different people, but blood is exactly the same color in everyone. It still doesn't look red. That's because we're seeing the *walls* of the veins too. When you see lemon- lime flavored soft drinks in plastic bottles they usually look green, but when you pour them out, they're often clear or yellow. It's not the blood that's bluish; it's the whole vein, including the walls, just like the soft drinks look green because they're in colored bottles.

Have you ever seen a totally white rabbit or mouse? They're called "albinos" because they can't make pigments (the substances that color our eyes, skin, and hair). Their veins look red, even though their blood is the same color as ours. Why is that?

From what I've seen, it is still red, but a different shade from oxygenated blood.

You could find out: prick yourself (with a CLEANED needle!) and squeeze out a couple of drops, and put them in water in a small cup. Then put the cup with the blood and water in it into a cooking pot (cup and all - don't spill it). Now, light a candle and put it in the pot. Close the lid of the pot and tape the lid all around the connection to the pot to seal it.

What you will have done is create an air-tight container with the candle inside of it burning. The candle will use up all of the oxygen inside of the pot and eventually snuff out. When this happens, untape the lid and look at the blood in your cup. It should be de-oxygenated.

Caveats:
1. Obviously, use a clean needle to prick yourself to draw blood. Sterilize it first! You don't want to get yourself sick by doing this experiment.
2. You need water in the cup because otherwise the water in your blood will evaporate, and blood changes color when it dries. Having water in the cup will mean that the blood is still wet.
3. You will need a good enough seal on the pot so that the candle can't get air from the outside. Otherwise, it won't snuff out.
4. You will want to do something to cool off the pot while you're doing this (especially the lid). Otherwise, the rising heat from the candle might warp the lid or something else that will be bad. My suggestion is to wrap a wet towel on top of the pot while the experiment is running. The wetness will prevent the lid from getting too hot.

There's a misconception that blood is blue when it is de-oxygenated, or without oxygen. This isn't true. Yes, blood changes its color a little bit when it is deoxygenated but at no point is it completely blue in your body. When you look at your arm your veins seem bluish-green because the light that bounces off other tissues and skin make them appear that way. So, in space your blood would still be red if you got cut, it may be a different shade of red, but it will still be red.

De-oxygenated blood in our body is very dark red.