[1] The bromine test is useful to confirm the result, although modern spectroscopic techniques (e.g. NMR and IR spectroscopy) are far superior in determining the identity of the unknown.
The formation of a red, blue, green, or purple coloration indicates the presence of phenols.
This complex has an intense colour, which may vary from blue, green or even red depending upon the nature of the phenol.
As an example using the chemical phenol itself: The ferric chloride test can be used to detect metabolites in urine in case of inborn error of metabolism such as phenylketonuria.
Also, it can be used to detect salicylates in urine, quick diagnostic test for aspirin overdose.