A method is provided for irreversibly coloring a coating composition containing a color former comprising the steps of a) providing a coating composition that contains an UV curable resin, a thermosetting resin or an alkyd resin with the proviso that the coating composition contains no developer nor any additive which may function as developer; b) mixing the coating composition with the color former whereby no color is formed; c) applying the colorless composition obtained in step b) to a substrate; d) in case of a thermosetting resin curing the resin between 100 and 300° C., preferably between 100 and 150° C., whereby no color is formed, in case of alkyd resin drying of the resin at room temperature, whereby no color is formed; e) exposure to UV or high energy radiation below 400 nm whereby the color is formed.