More Science » February 2010 Email Print See Inside Seeing Forbidden Colors People can be made to see reddish green and yellowish blue--colors forbidden by theories of color perception. These and other hallucinations provide a window into the phenomenon of visual opponency By Brian H. Tsou and Vincent A. Billock This is only a preview. Get the rest of this article now! Select an option below: Buy Digital Issue Customer Sign In *You must have purchased this issue or have a qualifying subscription to access this content It has been identified that the institution you are trying to access this article from has institutional site license access to Scientific American on nature.com. Click here to access this article in its entirety through site license access.