To permit, selectively, multi color prime printing or prime-and-verso printing, two rubber blanket cylinders (5, 6) each in engagement with a respective cylinder (3, 4) cooperate with a single printing or impression cylinder (7), and an engagement element (23, 24, 28) such as a roller (23, 24) or a compressed air supply (28); the rubber cylinder can be engaged with the impression cylinder or, respectively, separated therefrom. The impression cylinder carries a rubber blanket to receive printing information from one (5) of the blanket cylinders to effect verso printing during one operation phase, when the engagement element controls engagement of a sheet about the impression cylinder the other rubber blanket cylinder being engaged when the sheet reaches the printing line for verso printing; during a subsequent operation phase of the machine, the second rubber blanket cylinder (6) is separated from the impression cylinder to prevent smearing contact therewith, while the first rubber blanket cylinder (5) transfers printing information from its associated plate cylinder (3) to the rubber blanket on the impression cylinder. The sheet feed mechanism feeds a sheet only for every alternate operating phase. In normal operation, with a sheet fed for each revolution, the impression cylinder receives no ink and multi color prime printing can be effected by the two rubber blanket cylinders (5, 6). Double inking can be obtained by selectively lifting off both rubber blanket cylinders for alternate revolutions and feeding a sheet, only for every other revolution of the printing system, with, then, both rubber blanket cylinders engaging the sheet looped about the impression cylinder.