George III, indigo and the blue ring test

Analysis of George III's urine at the height of his insanity revealed a deposit in the shape of a blue ring, characteristic of indigo. But what could have been the source of indigo and was it linked to his dementia?

Most of us pay little regard to the colour of our urine. The various shades of yellow are caused by the presence of a variety of pigments - urobilins - the degradation products of haemoglobin. But other colours are possible: red, orange, and even green, purple, brown and blue. The first of these should necessitate a visit to the doctor. Blood tints the urine red, and needs early investigation. But what about shades of blue and green? Possibly the most famous report of blue-tinted urine is that of George III (1738-1820) just before he became too ill to rule. Does the colour give us any clues to the cause of his eventual fall from power?