Micrograph of melanosis coli, with the characteristic mucosal lipofuscin-laden macrophages (brown). Melanosis coli, also pseudomelanosis coli, is a disorder of pigmentation of the wall of the colon, often identified at the time of colonoscopy.It is benign, and may have no significant correlation with disease. The brown pigment is lipofuscin in macrophages, not melanin.urther information: Factitious diarrhea. The most common cause of melanosis coli is the extended use of laxatives, and commonly anthraquinone containing laxatives such as Senna and other plant glycosides.