As an astrobiologist, I often get asked how aliens would look. What people usually mean by “aliens” are complex, animal-like aliens—often humanoid in form.
From the natural history of life on our own planet, we already know that biology allows many, many potential body plans. We also know that the form and shape a complex organism takes—called the phenotype—depends largely on the environment in which it lives. Because there are so many unknowns, let me address one small part of the question, one that goes to the very heart of things: Would an alien organism necessarily have blood? And if so, what color might it be? My thinking was prompted by an article I dug up by Zachary Rodriguez and colleagues from Louisiana State University describing New Guinea lizards that have lime green blood.
Any complex life form, even on another planet, very likely would need a circulatory system to deliver nutrients to cells in its body and take away waste products. This is generally true for all of the more complex animals on Earth (although some very primitive animals such as sponges do not have blood). Even plants have a kind of circulatory system, with liquids moving throughout their “bodies.” We don’t think of it as blood, but when a plant is cut, it directs nutrients and minerals around the cut to seal off the injured area—not unlike blood clotting in animals.
Blood is deep in our psychology. We’re strongly affected when we see life’s fluid draining from our own bodies or the bodies of others. It may be no accident that the color for warnings and stop lights is red.
Most of our fellow vertebrates also have red blood due to iron-containing hemoglobin, which carries oxygen from lungs (or gills) to tissues in the rest of the body. One might think that any highly evolved organism would be the same, but that’s not necessarily so. For example, the blood of some octopuses—among the most intelligent species on our planet—is blue when oxygenated. Instead of hemoglobin, their blood uses copper-containing hemocyanin as its oxygen-carrying protein. Some spiders, horseshoe crabs, and scorpions also have blue blood.
In the universe of Star Trek, Vulcans and Romulans have green blood, while Andorians’ blood is blue. And of course, we say that noble-born humans have blue blood. Some of them might, actually, if they’re diagnosed with sulfhemoglobinemia. People who have been exposed to lots of sulfur-containing compounds sometimes have their skin turn a blueish tint, while their blood can turn dark blue, green, or black. This occurs because a sulfur atom is incorporated into hemoglobin (no worries if it happens to you—once your body clears the sulfur, your blood becomes red again).
As for the green-blooded lizards, according to Rodriguez the color comes from the bile pigment biliverdin. It may be an adaptation to fend off parasites. Other animals with green blood include earthworms and leeches, which have the oxygen-binding protein chlorocruorin in their blood plasma, and certain insects that take in pigments from plants.
In fact, it seems that nearly all blood colors are possible in principle. Ladybugs have yellow blood, lamp shells purple blood, and if you are living in a cold environment your blood can be translucent, because cold water can hold so much oxygen that hemoglobin becomes unnecessary.
In short, there is no single, definitive answer as to what color blood an alien might have. We can only say that any animal-like alien is likely to have it. But it could be of any color, depending on the organism’s particular biochemistry and the environment in which it lives.