A method for transforming three color-input signals (R, G, B) corresponding to three gamut-defining color primaries of a display to four color-output signals (R′, G′, B′, W) corresponding to the gamut-defining color primaries and one additional primary of the display, where the additional primary has color that varies with drive level, comprising: a) determining a relationship between drive level of the additional primary and intensities of the three gamut-defining primaries which together produce equivalent color over a range of drive levels for the additional primary; and b) employing the three color-input signals R, G, B and the relationship defined in a) to determine a value for W of the four color-output signals, and modification values to be applied to one or more of the R, G, B components of the three color-input signals to form the R′, G′, B′ values of the four color-output signals.