Firstly phthalic anhydride in presence of conc. H2SO4 undergoes protonation to give an intermediate carbocation. This carbocation reacts with 2-propylphenol in presence of NaOH to give pink colour compound. Mechanism
2-propylphenol gives pink colour on reaction with phthalic anhydride in conc. H2SO4 followed by treatment with NaOH.