A method of converting a three primary color input signal into a four primary color drive signal, includes defining three functions representing the first, second, and third drive signals as a function of the fourth drive signal, determining the intersection values of the fourth drive signal at a set of intersections of the three functions mutually, and of the three functions and a line defined by the fourth drive signal being equal to itself, determining boundary values of a valid range of the fourth drive signal in which all drive signals have valid values, determining calculated values of the first, second and third drive signals at the intersection values and the boundary values, calculating the maximum value (Vmax) or minimum value (Vmin) of the calculated values at the intersection, and selecting the intersection value at which the maximum value or minimum value is minimum or maximum, respectively.