Ideas don't have a noumen part and can't appear different from what they are. The error in your example is in association between the idea of zebra and an external object, the idea of zebra itself isn't wrong and isn't illusion and isn't different from some kind of true form of itself.
I am having difficulty understanding your point. Please feel free to clarify. To me "noumen" means mental or "thought" derived part.
Yeah the couch looks like a zebra. The idea it looks like a zebra is not wrong. However the conclusion that it therefore is a zebra is wrong. It is a visual misidentification. What distinguishes this from an illusion? We are not looking at true forms versus reality. We are looking at perceived form versus reality. When perceived form diverges from reality that seems like an illusion to me. Maybe you could call it something else, a mistake. What's the point here that you are trying to make?
