How to define which animal's blood is what color?

The colour of an animal's blood depends on the type of protein found in its blood cells which transports oxygen.

For example, in humans blood is red because our red blood cells contain haemoglobin, an iron containing pigment which is red. Other animals have green or blue blood (squid), because they have a different pigment on the blood cells which carries oxygen. You would have to look up which pigment each animal has and which colour it makes it blood.