In the field of colour printing, image data is derived by scanning an image to derive a set of colour-component signals for each image pixel.  Usually a black printer signal is derived from the colour-component signals and ideally the colour component signals are then reduced to take account of the contribution of the black printer signal.  It is normal to store the signals before using them to control apparatus for making the colour separations. 
It is known to display the stored image on a cathode ray tube forming part of a monitor and to modify the displayed image by altering for example its tone, colour balance and gradation; when a satisfactory displayed image is achieved, the modifications are applied to the stored data before that data is used to form images on the colour-separation films or plates.