This is an interesting question and one that has been surrounded in a bit of controversy due to a myth that has been circulating (get it...circulating) for a long time. The myth is that... that blood is red when it is filled with oxygen and blue when it does not have any oxygen in it. This myth is completely false, and don't believe anyone who tells you otherwise. I've known well educated adults to be fooled by this one and it simply is not true.

The truth is this... as we all know, blood is what carries oxygen to your various parts of your body. It receives oxygen from the lungs, flows through the body and then delivers this oxygen where it is needed before making the round trip back to the heart where it is passed again to the lungs. The oxygen dissolves in your blood, binding to the red blood cells.

Human blood is always red. The only difference is that when it is oxygenated, it is a bright red, and when it is depleted of oxygen, it is a darker red. The myth of blue blood may have several origins,
1) veins, which carry the blood once it is low on oxygen, look blueish green, but that's because of the tissue that makes up the veins and is not due to the blood itself.
2) there is a term "blue blooded" which, especially in previous eras, used to refer to someone who was of royal or noble class. So, if your family was rich or notable, people may have said that you had "blue blood".

Despite these things, nobody's blood has ever been blue. You can double check this the next time you are at a doctor's office and need to have a blood sample taken. If you are brave enough to look at the blood being drawn, it is notable dark red, it is the blood from your veins that is low in oxygen.

If you want to know more, try this on for size... the red color of our blood is red because our cells contain tiny amounts of iron (which is contained in a molecule called hemoglobin which you may have heard of).

Iron turns red when it combines with oxygen, just like when a piece of metal rusts, it turns a red-ish orange. It is also the color of Mars, the "red planet" for exactly the same reason. The dirt on Mars has a lot of iron. Some animals though, use a different mechanism to transport oxygen. Instead of using iron, they use copper. Copper also likes to bind to oxygen. If you've ever seen an old historic building with the green-ish blue roofs... those are copper... well, they used to be, now they are just rusted copper. So copper turns green-ish blue when it combines with oxygen. And so animals that use copper to transport oxygen have... you guessed it... blue blood! Most notable, lobsters, spiders and snails have blue blood. But certainly not humans. I hope that answers your question, and maybe makes you curious about a whole much more.

Blood that is leaving the heart and going to the rest of the body has just recently been oxygenated, and is therefore red. Blood heading back to our hearts after making a trip around the rest of the body has released the oxygen (i.e. is de-oxygenated) and is a darker red.

Our blood inside our bodies is red, same as it is outside - the color is due to iron oxide (rust) which is attached to a protein called hemoglobin. You can tell this by holding your hand up to a light and looking through it - you will see a red glow, the color of your blood filtering all of the non-red light from the light source beyond your hand.

Blood is red, just like you see when you have a cut. It’s a dark red when it is not carrying a lot of oxygen. It is a brighter red when it is full of oxygen. What we see when we look at our veins depends on things like our skin color. The vein is made of connective tissue that is bluish white. Then we have layers of skin cells. Most of us have some pigment or color in our skin. Some have a little, some have a lot. All of this changes what we see.

If you looked at a green soda bottle, you might not know that the soda inside was clear or yellow. It is the same with our blood. If you look at the veins of a person with pale skin, the blood looks blue. If you look at the veins of a person with dark skin, it looks very dark. But all of our blood is the same color.

What does blood do for us besides carry oxygen around? It has many different jobs. See how many you can imagine.

If you are interested in things like blood, you may want to study physiology.