The present invention provides a colorimetric process for the detection of an oxidative coupling reaction in which a coupling component is reacted with a developer component in the presence of an oxidation agent to give a colored material, wherein, as coupling component, there is used a compound of the general formula: ##STR1## in which X is a bromine or chlorine atom, n is 1, 2 or 3, Y is COOR, CONRR', SO.sub.3 R, SO.sub.2 NRR', OR, NRR' or NRR'R".sym. and R, R' and R", independently of one another, are hydrogen atoms or C.sub.1 -C.sub.3 -alkyl radicals.The present invention also provides a reagent which contains the above compound, as well as new substituted phenols and a process for the preparation thereof.