Apparatus for use in calibrating a display system comprising digital-to-analogue converters (D.A.C) (11,12,13) for converting a digital colour control signal to analogue colour component signals which are suitable for controlling an electronic display. An analogue signal modifier modifies the signals from the D.A.C. The apparatus also comprises a switch (10) for connecting between the D.A.C. and the analogue signal modifier. The switch is operable in a first mode (52,54) to pass signals from the D.A.C. (11,12,13) directly to the analogue signal modifier or in a second mode (53) to pass analogue calibration colour component signals to the analogue signal modifier such that the appearance of the signals from the D.A.C. and the calibration signals can be visually compared on a screen (17) to determine whether any reproduction errors are in the D.A.C. (11,12,13) or in the signal modifier.