The color of the stool of patients with ulcerative colitis is mainly determined by the location of the colitis lesion. If the ulcer occurs in the rectum or sigmoid colon, which is closer to the anus, the bleeding usually occurs quickly and the color of most stools is bright red or dark red blood, which may be accompanied by blood clots or blood clots, and may be accompanied by pus or mucopurulent blood stools. If the ulcer is deep, or at the end of the ileum and the ascending or transverse colon, the stool stays in the intestine for a longer period of time and the trivalent iron ion hemoglobin easily turns into divalent iron ion, so the color of the stool is mostly dark red or black, and some may have tarry black stool. Therefore, the color of the stool of patients with ulcerative colitis is determined by the location of the ulcer, the length of time the stool stays in the intestine, and the degree to which the trivalent iron ions of the blood turn into divalent iron ions.