A method for operating an offset printing machine printing at least one of the process printing colors cyan, magenta, yellow and black and optionally at least one further printing color, wherein the at least one printing color are applied to a printing material to be printed via at least one printing unit of the offset printing machine in such a way that the respective printing unit has a plurality of ink zones, into which the respective printing color is applied in individual zones to the printing material to be printed, with at least the following steps: full-tone measuring fields of at least one of the process printing colors cyan, magenta, yellow and black printed onto the printing material in ink zones of the printing material are measured by means of a sensor, wherein a full-tone measuring field is measured by means of the sensor in each ink zone (z) of the printing material for the respective process printing color in such a way that a main density measurement value and several secondary density measurement values are detected for the respective full-tone measuring field in a measuring ink space, in the ink zones of the printing material, white measuring fields arranged adjacent to the full-tone measuring fields are furthermore measured by means of the sensor, wherein several paper white density measurement values are detected in the measuring ink space for each white measuring field, for each of the ink zones of the printing material, that white measuring field with the greatest paper white density measurement value (Y rgbv (z)) is determined from all paper white density measurement values of all white measuring fields of the respective ink zone (z), wherein the paper white density measurement values of the white measuring field with the greatest paper white density measurement value (Y rgbv (z)) are stored for the respective ink zone (z), with the help of a polynomial regression for the colors red (r), green (g), blue (b) and black (v) of the measuring ink space, a course of a paper white value (Ŷ rgbv (z)) across all ink zones is in each case determined from the paper white density measurement values stored for all ink zones of the printing material of the respective white measuring fields with the greatest paper white density measurement value (Y rgbv (z)), for each of the ink zones of the printing material, the paper white density measurement values of the respective white measuring field with the greatest paper white density measurement value (Y rgbv (z)) are corrected (Y rgbv (z) - Ŷ rgbv (z)) by the paper white value (Ŷ rgbv (z)) determined for the respective ink zone (z) and the respective color of the measuring ink space, for each of the ink zones of the printing material, a quotient is formed from the minimum and maximum paper white density measurement value for the corrected paper white density measurement values of the respective white measuring field with the greatest paper white density measurement value, depending on this quotient, a conclusion is drawn as to whether an unwanted scumming is present in the respective ink zone (z) of the printing material. full-tone measuring fields of at least one of the process printing colors cyan, magenta, yellow and black printed onto the printing material in ink zones of the printing material are measured by means of a sensor, wherein a full-tone measuring field is measured by means of the sensor in each ink zone (z) of the printing material for the respective process printing color in such a way that a main density measurement value and several secondary density measurement values are detected for the respective full-tone measuring field in a measuring ink space, in the ink zones of the printing material, white measuring fields arranged adjacent to the full-tone measuring fields are furthermore measured by means of the sensor, wherein several paper white density measurement values are detected in the measuring ink space for each white measuring field, for each of the ink zones of the printing material, that white measuring field with the greatest paper white density measurement value (Y rgbv (z)) is determined from all paper white density measurement values of all white measuring fields of the respective ink zone (z), wherein the paper white density measurement values of the white measuring field with the greatest paper white density measurement value (Y rgbv (z)) are stored for the respective ink zone (z), with the help of a polynomial regression for the colors red (r), green (g), blue (b) and black (v) of the measuring ink space, a course of a paper white value (Ŷ rgbv (z)) across all ink zones is in each case determined from the paper white density measurement values stored for all ink zones of the printing material of the respective white measuring fields with the greatest paper white density measurement value (Y rgbv (z)), for each of the ink zones of the printing material, the paper white density measurement values of the respective white measuring field with the greatest paper white density measurement value (Y rgbv (z)) are corrected (Y rgbv (z) - Ŷ rgbv (z)) by the paper white value (Ŷ rgbv (z)) determined for the respective ink zone (z) and the respective color of the measuring ink space, for each of the ink zones of the printing material, a quotient is formed from the minimum and maximum paper white density measurement value for the corrected paper white density measurement values of the respective white measuring field with the greatest paper white density measurement value, depending on this quotient, a conclusion is drawn as to whether an unwanted scumming is present in the respective ink zone (z) of the printing material.