Colours provide a paradigm example of how one can introduce sceptical challenges to our ordinary beliefs about the world. These sceptical challenges come in at least three forms: the Pyrrhonian challenge, the traditional problem of the external world, and the problem of acquaintance. The traditional problem of external world can arise from the split between the primary qualities of objects and the secondary qualities of objects. Recall that the idea is that a property like being the colour red is too differential in subjects’ experiences to be among the primary qualities. A related epistemological worry arises from the thought that our knowledge of the world is not simply propositional knowledge, or what Bertrand Russell called our “knowledge of truths”. However, as Mark Johnston argues, as far as acquaintance knowledge of world goes, it’s hard to see how agents in good case are any better off than the agents in the bad case with respect to acquaintance knowledge of the world.