Infographic on the chemistry of the colours of blood. Human blood is red due to haemoglobin, which is also red when deoxygenated - not blue! Spiders, crustaceans, and molluscs including octopuses and squid have blue blood due to haemocyanin, which contains copper instead of iron. When deoxygenated, haemocyanin is colourless. Some segmented worms, some leeches and some marines worms have green blood due to chlorocruorin, which is light green when deoxygenated. Marine worms including peanut worms, penis worms and brachiopods have purple blood due to haemerythrin, which is colourless when deoxygenated.