Certain vitamins such as Vitamin C and B vitamins are water soluble. If the human body receives excess Vitamin C and Riboflavin (also known as Vitamin B2), urine will turn a bright, sometimes neon, yellow-orange. Although the color of your urine shows that your body is absorbing and eliminating nutrients, it is not necessarily an indication of the efficacy of a vitamin product.  Water soluble vitamins (such as vitamin C or Riboflavin) are flushed from the body on a daily basis because the body doesn’t store them; whatever your cells don't use and need of any particular water-soluble vitamin is excreted in urine.