Most food colours, including the colour in juice are pigments that go through a different excretion method than through the urine. remember urine is an excretion process to rid the body of nitrogenous waste products. most pigments in food colour are not nitrogenous. Most likely the pigments in food colour will be excreted in feacal waste. Also the amount of food colour is usually very little and is diluted out in all the water and other food you eat. So unless you drink litres and litres of red juice then maybe feaces will have some traces of the colour.