Iron (III) Chloride reacts with phenols forming octahedral iron (III) complexes. Blue, red, green, or purple coloration indicates presence of phenols.
Test is performed at low Iron (III) Chloride concentrations usually in aqueous solution, if the sample is insoluble in water DCM + few drops of pyridine might be used as a medium.
It is worth mentioning that other substances that are significantly present in enolic form and also hydroxamic acids, oximes, and sulfinic acids undergo this reaction as well.