Phenolphthalein (C20H14O4) is a useful compound to industrial water treatment. While colorless in the pH range of 0-8.2, it is a pink to fuchsia in the 8.2-12.0 range (going colorless again at > 13.0).
This property is exploited for alkalinity titrations where acid is added to bring the pH of an alkaline solution down to 8.2 when the solutions turn from pink to clear (making the approximate point where carbonate alkalinity is fully converted to bicarbonate alkalinity). Phenolphthalein can also be used as a rough pH indicator to confirm questionable pH probe measurements.
Another use is as a dye in closed loop water systems. Typically maintained at pHs above 8.2, the closed loop water is pink and easily identifiable if leaks occur.