Your kidneys have two important goals, and that is to excrete toxins and balance water. There are always similar amounts of urea in the body, but the amounts of water vary.

When you drink too much water, your blood gets a bit watery, and your pituitary stops making vasopressin, also known as Anti-diuretic hormone. In layman's terms, this hormone tells kidneys to reabsorb water, and if it isn't there, your kidneys send the water into your bladder instead of back into your blood. So the amount of colorants is nearly constant, but their dilution changes, and that's what you can see when you drink too much or little.