How do you determine which animal's blood is of what color?

Before seeing an animal for yourself, predicting the color of its blood can be a very difficult and tricky task. Animals have developed a myriad of molecules and methods to survive in their environments, leading to a rainbow of blood colors.

Red blood is probably the most common and is caused by a molecule called hemoglobin. This molecule (found in our red blood cells) helps hold onto oxygen and better spread it around the body. An atom of iron in each molecule results in the red shape. A vast majority of animals use this same molecule in order to help facilitate the transfer of oxygen around their body. 

Blue blood is possible as well and can be found in several species of octopus. These octopuses use a different molecule called hemocyanin, which uses copper instead of iron but works for the same purpose: to carry oxygen to different cells around the body. But notice this is not in all species of octopus, but it does seem to help the cold water ones more than the warmer water cousins.

Cold water, however, is not always a predictor of blood coloring. One fish, the ocellated ice fish, requires no molecule to help carry oxygen, as the frigid waters it inhabits are extremely oxygen rich, and has clear blood. 

One skink in Papua New Guinea has developed a very unique green blood coloring. The green-blooded skink (aptly named) uses hemoglobin like any red-blooded creature does with one minor difference. As hemoglobin is used and wears down, we destroy it, creating the waste molecules bilirubin and biliverdin. In our bodies these are released into the small intestine for removal, but in the skink the biliverdin is kept in high amounts. If we did not remove this molecule it could cause jaundice and death in humans. This increased biliverdin level turns the skink's blood green and, according to one hypothesis, helps to protect the skink from malaria or other diseases. 

In conclusion, with a sampling of blood from the organism, the blood color would be very easy to read for. With no sample, however, it can be very difficult; every species is unique and has evolved its own systems for survival that can be hard to predict. Your best bet would be to compare the animal to members of its close family or genus and make a prediction from there. I hope this helped answer your question!