The redness must be in the monad. The point of postulating monads is to have an ontological correlate to the phenomenological unity of consciousness. Redness is interior to consciousness, which is interior to the thing that is conscious. A theory of monads might have an intermediate stage of incomplete descriptions, in which it is described purely formally, mathematically, and causally, but the objective is to have a theory in which there is something recognizably identical with a total instantaneous conscious experience. This is also the point of reverse monism.