The knowing of an Appearance as an object is constituted by its sensorial awareness of that very appearance. So that the knowledge of the cognized Appearance or Form appears as the being-conscious of that Appearance or Form, its apperception. Form therefore is composed of its sensorial awareness, or qualities, as Appearance, when it is cognized as Vijnana, its Consciousness, the apperceiving act. Therefore Vijnana is not conceivable as such, itself the rendering of Appearances as actual and realizable.
Neither of the skandhas ”do” anything, they are basically interpretations of noumenality. Form-Consciousness are not objects in themselves, they are functional appearances, the very appearances (form, feeling, perception, etc) being the functioning that they are. As objects they are conceptual.
"Sariputra, in emptiness there is no form, nor feeling, nor perception, nor impulse, nor consciousness".
The only experience we can have of Salt is its Saltiness. So that Saltiness is the experience of the concept we have chosen to call Salt. But Salt (experienced via the skandhas) cannot be separated from the sensorial expression of it, i.e., Saltiness, because without its "Saltiness", we wouldn't call it Salt in the first place.
”Saltiness” here, suggests all the sensorial qualities we ascribe to it, i.e., ”the six consciousnesses”, via the skandha-mechanism, so that the resulting ”form” is what we call Salt.