- Purple urine bag syndrome (PUBS) is a rare condition, in which the patient has a purple-colored urine tubes or bags following urinary catheterization for hours to months. It is usually associated with elderly females, prolonged urinary catheterization, some bacterial infections that produce sulphatase/phosphatase, chronic debilitated state, and chronic constipation.

Biochemical pathway of conversion of tryptophan to indigo and indirubin involved in the development of purple urine bag syndrome.
Sources:
http://synapse.koreamed.org/DOIx.php?id=10.4235/jkgs.2012.16.4.221&vmode=PUBREADER#!po=50.0000