Does Watermelon Juice Cause Reddish Stool?

Can watermelon juice (8 to 16 oz. per day) cause a child s stool to have a strong reddish cast---not through and through solid red, but a definite reddish appearance? My grandchild has episodes of severe constipation that can result in bright red rectal bleeding (slight, not copious) when he finally ( very painfully) passes stool. But it has occurred to me that the typical reddish cast to his stool, when he is not constipated, may not be due to the watermelon juice he drinks but to actual intestinal bleeding as in colitis. I am trying to determine if we are possibly missing a more serious diagnosis by attributing the red cast of his stool simply to all the juice he drinks. If we were to try a trial period of no watermelon juice, how long should the trial period last? A week? We give him daily Miralax to control the constipation per a gastroenterologist s directions but the parents are not together and the other parent is not always compliant---if at all--and we still have a very constipated child on occasion. Thank you.