The fluorescent yellow color of urine after taking certain vitamins is often due to the presence of excess riboflavin (vitamin B2) in the body. Riboflavin is a water-soluble vitamin that is naturally fluorescent and can give a bright yellow color to urine when it is present in large amounts. When you consume a vitamin B complex supplement or a multivitamin that contains riboflavin, your body absorbs the necessary amount for its functions, and any excess is excreted through urine. Since riboflavin is not stored in large quantities in the body, the excess is quickly eliminated, resulting in the vibrant yellow color. It's worth noting that the fluorescent yellow urine colour is harmless and does not indicate a health concern.