When your body metabolizes vitamins they are used by the body and eliminated and when eliminated the urine will turn color. If a vitamin is not absorbed at all it will simply leave the body via the stool, never changing the color of the urine. The fact that the urine changes color simply denotes that it was absorbed, filtered by the kidneys and released via the urine. Before elimination the nutrients are in the blood and utilized for thousands of metabolic process before it's release. The color of the urine has no correlation on how valuable it was in the body before release. In fact without any change in color, I would question absorption.
Dr. Joe Esposito