Purple urine bag syndrome (PUBS) was first reported in 1978. PUBS is rare, occurs predominantly in constipated women, chronically catheterized and associated with some bacterial urinary infections that produce sulphatase/phosphatase. The etiology is due to indigo (blue) and indirubin (red) or to their mixture that becomes purple. A chain reaction begins in the gastrointestinal tract with tryptophan as described in the article.
Duff, M. L. (2013). Case report: Purple urine bag syndrome. Journal of Emergency Medicine, 44(4). https://doi.org/10.1016/j.jemermed.2012.11.030