Respiratory pigments are metalloproteins that transport O ₂ , the best known being the bright red/crimson coloured hemoglobin in human blood. The colour derives from Fe ²⁺ at the core of a tetraporphyrin ring. But less well known is blue blood , and here the colour derives from an oxyhemocyanin unit based on Cu ¹⁺ (the de-oxy form is colourless) rather than iron.