Abstract: A key color control system for a multicolor rotogravure printing press maintains a constant actual base printing repeat length at the key color printing deck. A control system for a printing press comprises means to detect and measure the magnitude of a repeat length error as it is printed by a key color deck and further comprises means to correct for the repeat length error by varying the tension on the web ahead of the key color deck. The system includes means for establishing high and low limits within which repeat length errors of certain magnitude are likely to occur and for which correction is made. The system also includes means for warning the press operator that there is a trend in the change in magnitude of repeat length errors to exceed the limits so that the press operator may either establish new limits or establish a new base repeat length within the same limits so that repeat length errors of a different magnitude (usually smaller) are likely to occur.