Your blood is red. That's the case for most animals on Earth.
But there are exceptions. Certain types of marine worms found in shallow ocean waters around the world have green and purple blood. And the common horseshoe crab has bright blue blood that gets harvested for bacterial-testing kits used by pharmaceutical companies.
This chart, made by chemistry teacher Andy Brunning at his blog Compound Interest, explains exactly why this variation occurs:
Our blood is red because of a protein called hemoglobin. This is the molecule that binds oxygen and allows your red blood cells to carry it throughout your body and supply it to your other cells, and its red because of the iron at its center. As Brunning points out, your blood is red whether it is carrying oxygen or not — diagrams show deoxygenated blood as blue simply for convenience's sake.
That's not the case, however, for horseshoe crabs:
These arthropods' blood uses a different protein, called hemocyanin, to bind oxygen. Because that binding process involves an atom of copper, rather than iron, the blood has a blue appearance when it's oxygenated, and little or no color when it's not.
Entirely separate from the color, horseshoe crab blood also has a chemical called coagulogen that can detect bacterial contamination at extremely low concentrations. It's very useful for testing pharmaceuticals, which is why companies collect horseshoe crabs and bleed them alive, as shown in the PBS documentary Crash.
A number of other marine species also have hemocyanin in their blood, and depending on the surrounding tissue, it can sometimes appear purplish, like in this red rock crab:
Like hemocyanin, it only turns color when oxygenated, and in some species — like the peanut worm — you can see this purple color without cutting them open, thanks to somewhat translucent skin:
Chemically, it's similar to hemoglobin, and uses iron to bind oxygen. And though it looks red when concentrated, when it's more diluted, it can appear bright green (and, as with peanut worms, can be seen through the skin):
Read more: Andy Brunning takes on another interesting chemistry question: Why does old book smell have hints of vanilla?