Humans have red blood. This is caused by the protein Haemoglobin. Most vertebrates have this type of blood because it is the most effective at transporting oxygen. Haemoglobin contains iron, which allows oxygen to "bind" with it.
Octopi, spiders, and squids have blue blood because of Haemocyanin in their blood. This has a different atomic structure that contains copper atoms instead of iron atoms.
Some worms and leeches have green blood because of a protein called Chlorocruorin. The protein is similar to Haemoglobin except it has an aldehyde group in place of a vinyl group in the chemical structure.
Some marine worms have violet blood, and this is because of Haemorythrin. Haemorythrin contains individual units that contain iron atoms.
Some animals have yellow blood because of the pigment,Haemovanabine. Haemovanabine is a vanadium-based compound.
I was wondering if an intelligent being, as in sapient and sentient, could have one of the other proteins besides Haemoglobin. The most likely are either Haemocyanin(blue), because it is the second best oxygen transporter, or Chlorocruorin(green), because it is only one compound different from Haemoglobin(maybe those movies were right about alien blood).
Octopi are pretty intelligent, as in brain function, but they are aquatic, and more blue-blooded creatures are in the water. Green blood is interesting, but only some worms have it. Violet blood would be really cool, but it is a combination of the last two, aquatic worms. Yellow blood is interesting, but apparently we don't know if the compound that makes the blood yellow transports blood.
Marine life is a weird field with many different blood colors. I want to narrow my question down to only terrestrial creatures because the majority of them have red blood.
Information came from these sites.