an image processing system is arranged to convert data representing a picture element with an original color defined in terms of original brightness and hue values into data representing the picture element with a new color , different than the original color . in the system a new hue value is calculated from the original hue value by way of a transforming circuit 5 . the original and new hue values are both processed in look up tables 4 , 6 to determine respective inherent brightness values associated therewith . the original brightness value is processed with the inherent brightness values by way of a subtracter 7 and an adder 8 to produce a new brightness value that together with the new hue value defines the new color of the pixel .