Epiphenomenalism is the view that some ideas are epiphenomenal - that some ideas come about in the brain but have no effect on the physical world. This is closely related to the debate between materialists and idealists.
According to Stanford, "The central motivation for epiphenomenalism lies in the premise that anything that can causally contribute to a physical event must itself be a physical event. If a mental event is something other than a physical event, then for it to make any causal contribution of its own in the physical world would require a violation of physical law."
The original argument that Jackson devised in Mary's room (explained in the Philosophy Bites podcast 'What Mary Knew') to show that materialism is inadequate to explain the way the world is concerned Mary, who even though she knew everything about the colour red in physical terms without actually having perceived it, would in effect not know what seeing red was actually like.
So the perception of red, or at least the qualia red, is a mental activity which is unrelated to physics in a sense.
So the red rose isn't really red at all (under a physicalist view), it is only when we perceive it that we get the qualia red in our mind (the idea). To tie this into Locke, under an epiphenomenalist view the rose doesn't cause the perception of red as Locke argued.