Objects appear to be a particular color because they reflect some wavelengths more than others.
A red apple is red because it reflects rays from the red end of the spectrum and absorbs rays from the blue end. A blueberry, on the other hand, reflects the blue end of the spectrum and absorbs the red. Thinking about Newton's discovery in 1802, the physician Thomas Young, who later helped decipher the hieroglyphics of the Rosetta Stone, concluded that the retina could not possibly have a different receptor for each of these wavelengths, which span the entire continuum of colors from violet to red. Instead, he proposed that colors were perceived by a three-color code. In 1964, George Wald and Paul Brown at Harvard and Edward MacNichol and William Marks at Johns Hopkins showed that each human cone cell absorbs light in only one of these three sectors of the spectrum.