Hello, Check your magnesium levels. Updating your blood work is recommended as an excellent way to check for deficiencies or excesses that can be responsible for a color change. Your hemoglobin and blood oxygenation level can be a factor. Concern is greater for a black, tar like stool than just a change to a darker color with increased iron intake. Make that it is not excessive. Blood in stool and color change can be an indicator of intestinal bleeding or an ulcer. Consult with your physician if additional diagnostic testing is needed such as a colonoscopy. Medicine interactions should be reviewed with your doctor before taking any supplements including vitamins. Certain disorders of the mouth can often produce red tissue in the oral cavity. Your dentist can recheck your tongue once you have checked your systemic function. Thank you for your inquiry.