A method of converting a three-primary input color signal (IS) comprising three input components (R, G, B) per input sample into an N-primary color drive signal (DS) comprising N≧4 drive components (D1, . . . , DN) per output sample for driving N sub-pixels (SP1, . . . , SPN) of a color additive display, the N sub-pixels (SP1, . . . , SPN) having N primary colors, the method comprises:
- adding (10), to three equations defining a relation between the N drive components (D1, . . . , DN) and the three input components (R, G, B), at least one linear equation defining a value for a combination of a first subset of the N drive components (D1, . . . , DN) and a second subset of the N-drive components (D1, . . . , DN) to obtain an extended set of equations, and
- determining (10) a solution for the N drive components (D1, . . . , DN) from the extended set of equations.