Hi,
You can try using a dimmable effect for the P to at least make it respond to the master intensity and blackout. To do so, create a fixture template with a RGBWA attribute and then a dimmable effect. Ideally, the fixture would support HSL or HSI like some ETC fixtures do. This way, the fixture knows how to best use all its color components to achieve the desired HSL. At the moment, these are your best options.