The ingredient most likely to make urine appear yellow is Riboflavin (Vitamin B2). Riboflavin is a water-soluble vitamin, meaning any excess is excreted in the urine. Riboflavin has a naturally bright yellow colour, and can cause urine to take on this yellow or sometimes neon yellow hue. This effect is harmless and simply reflects the body's elimination of surplus riboflavin, this is very common when taking B-complex vitamins.