You encounter some fascinating stuff reading the New England Journal of Medicine. Take NEJM’s Image of the Week this week — yes, yes in fact that is a bag of purple urine. Since you asked:
Purple discoloration can occur in alkaline urine as a result of the degradation of indoxyl sulfate (indican), a metabolite of dietary tryptophan, into indigo (which is blue) and indirubin (which is red) by bacteria such as Providencia stuartii, Klebsiella pneumoniae, P. aeruginosa, Escherichia coli, and enterococcus species. The clinical course is benign, and the urine typically clears with resolution of the bacteriuria and acidification of the urine.
Link and image via NEJM.
This looks similar to Porphyria, which is what Mad King George III is believed to have suffered from.