Urine color is not a good test as it is subjective and food such as blackberries, medications and vitamins can change the color of your urine.
The most accurate predictor of water lost during a workout is simple, weight loss.
1. Weigh yourself before workout, as little clothing as possible
2. Weigh yourself after workout, again as little clothing as possible.
If you lost >2%, your sweat is not accurately replacing fluid. For every kilogram of weight lost, the NSCA recommends 1.5 liters, or 50 oz., of water with some potassium and sodium as well as carbohydrate of 5-10% concentration.