A common migraine medication has an uncommon side-effect: it can turn blood green. People with urinary catheters can sometimes produce purple pee.
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Purple urine bag syndrome occurs when bacteria produce an enzyme that reacts with urine. The result are two natural dyes, indirubin and indigo, which combine to turn someone’s pee purple. This is also gross, but it is apparently harmless.
And while we’re on the topic of urine colour, there’s a persistent myth that there’s a special kind of dye you can add to a pool to track down rogue pool-urinaters. This is alas not true, and not really viable either, because the compounds in urine are pretty much immediately purged by the chlorine.
Methylene blue notoriously freaks people out when they pee. As do beets in their different way.
A friend of mine worked at a hospital in the testing lab. He told me once that he had seen every colour of faeces. I asked him, incredulously, “every colour?” He nodded, sadly.