If phenolphthalein solution is added to an ammonia-based window cleaner, the solution would turn pink. This is because phenolphthalein is a pH indicator, and it changes color depending on the pH of the solution. Ammonia is a base, and it has a high pH. Phenolphthalein turns pink at a pH of about 8.3, so when it is added to an ammonia-based window cleaner, the solution will turn pink.