Blood and its color(s)
We, as most mammals, have red blood. This is due to the presence of iron in our blood, which when oxidize turns reddish.
But why is this iron there?
Well we all know we need oxygen in our body, that’s why we breathe, but we may not realize that most of our cells requires oxygen to live, all over the body.
In order to propagate oxygen to our body we need hemoglobin, it will provides a mean to transport oxygen throughout our tissues so our cells can stay alive.
So there is iron, and there is oxygen, which lead to redox, hence the red color.
But my veins are blue
Yes, we do see them blue, but its not blue. Some people will tell you otherwise, but blood is never blue in human. Why we do perceive the veins blue does seem to be a question without perfectly proven answer.
One of the most likely reason is the light and its energy that the blood and skin will reflect and absorb. If you want to know more about this particular topic, I invite you to read this article which covers the topic pretty well, while providing more interesting links on the subject.
Why is this post titled color(s)?
We do have have red blood, but there are other blood colors that exists, not in humans through.
Bluish blood
Lobsters, squids and horseshoe crabs (amongst others) have a bluish blood, as their blood contains hemocyanin where we have hemoglobin, and use copper (vs iron in human) as a medium of transport.
You can have a look at pictures of horseshoe crabs and their blood, but while you’re at it read more about the use of their blood to test drugs, and how it probably saved a lot of people!
Green blood
The skink lizard have green blood. Apparently, it seems that a high level of bile molecules are present in their blood, but also in other tissues, and even in bones apparently.
More on this on National Geographic
Yellow blood
Insects such a beetles are known to have yellow colored blood. Really, pure blood should be clear in insect, they get color because of what is in it.
Insects don’t have lungs and the need to transport oxygen like us, they breathe through their skin, and for most parts, blood does not carry oxygen and therefore does not have hemoglobin on its equivalent, but it does carry nutrients and hormones, so most of the time, their blood color is related to what they have been eating.
There is a bit more in here.