A method is provided for operating an image generator, arranged to generate and to output signals for displaying image elements of a predefined set of image elements, each image element being defined by respective varying deflection signals. A colour change trigger is firstly defined comprising an additional image element defined by varying deflection signals different to varying deflection signals of an image element of the predefined set. Varying deflection signals of the defined colour change trigger are generated and output thereby to indicate a respective colour for use in displaying an image element of the predefined set.
An image generator is also provided, configured to implement the method. A method is also provided for generating digital colour image data from signals output by such an image generator. A display system is also provided incorporating such an image generator together with a colour change trigger detector and scan converter.