... what is interesting as well is the colour of the mucus produced when you have cold. Especially during sinus infection, large amount of green snot is produced. But the mucus should be colourless, so where does this come from? The answer is, that it contains dead white blood cells - a result of a fight your immune system is waging against the infection. The colour itself is caused by haem (the same chemical, which carries iron and binds oxygen in haemoglobin). As a quite conjungated aromatic system, haem derivatives are often deeply coloured. What is really cool, is that the haem in the snot is mostly contained in an enzyme, which produces hypochlorite ions by oxidation of chloride (haem is usually contained in redox-active enzymes). So your body produces its own "bleach" (active agens in bleach is the hypochlorite) to disinfect your innards :)