Abstract: The lower side of a transmitter sheet (1) of a copy form set of chemically reacting color reaction paper provided with a CB coating (2) is zonewise provided with an additional ink layer (4) e.g. on wax basis. On writing or printing upon the copy form set within the intensification zone (3) defined by the additional ink layer (4) the color pigments of the additional ink layer (4) are partially or completely dissolved by means of the CB substances of the CB coating and transmitted to the CF coating (5) of the receiver sheet (6). This results in that the copy is itensified within the area of the intensification zone (3), thus becoming particularly machine-readable.