As far as I know, having tried similar things over the years, the only way to do that on a PLT texture would be to make those parts of the texture SO pale they just look white, regardless of what colour is actually occupying that channel... which tends to look BAD.
PLTs default to "skin" channel, unless you tell them otherwise. There's no "nothing at all" channel where you can leave texture areas uncoloured.
Shabby Hyena Item returns to the perennial "not enough channels" gripe...