I recently received this question via email, so here's the answer. No. There is no chemical which changes color when someone urinates in a swimming pool. There are
dyes which could cloud, change color, or produce a color in response to urine, but these chemicals would also be activated by other compounds, producing embarrassing false-positives.
Though there is no such thing as urine-detecting dye, you can purchase signs that prey upon the misconception that a urine indicator exists. The signs, which warn the pool is being monitored with a chemical 'wee alert', are believed to be an effective deterrent against urination in a pool, particularly with adult swimmers.