King George III had an illness that caused his poop to turn purple!

Porphyrias are a group of rare inherited or acquired disorders of certain enzymes that normally participate in the production ofporphyrins and heme. They manifest with either neurologicalcomplications or skinproblems, or occasionally both.

Porphyrias are classified as hepatic or erythropoietic based on the sites of accumulation of heme precursors, either in the liver or bone marrow and red blood cells.

The term "porphyria" is derived from the Greek πορφύρα, porphyra, meaning "purple pigment".

The name is likely to have been a reference to the purple discolouration of feces and urine when exposed to light in patients during an attack.

King George III suffered from one of these rare inhertited disorders and hence his poop was always purple.

(Source)