Phenolpthalein is colorless in acidic solutions. Phenolpthalein is a chemical commonly employed as a pH indicator. It turns pink in basic solutions that have a pH above about 8.3; as a solution becomes more basic, it turns fuchsia.
Phenolpthalein itself is a weak acid, so it produces hydrogen ions in solution. It turns pink in basic solutions because the hydroxide ions in those solutions deprotonate, or remove hydrogen ions from, phenolphtalein. Deprotonated phenolpthalein is pink. Phenolpthalein is not soluble in water, so it must be dissolved in ethanol or another solvent for use as an indicator. Phenolpthalein is also sometimes used as a laxative.