Hermann von Helmholtz finally brought to an end both the confusion of additive versus subtractive colors and the confounding of the perceptual phenomenon of color with the objective phenomenon of light spectral composition.
Luminous rays of different wave--length and colour distinguish themselves in their physiological action from tones of different times of vibration, by the circumstance that every two of the former, acting simultaneously upon the same nervous fibres, give rise to a simple sensation in which the most practised organ cannot detect the single composing elements, while two tones, though exciting by their united action the peculiar sensation of harmony or discord, are nevertheless always capable of being distinguished singly by the ear. The union of the impressions of two different colours to a single one is evidently a physiological phaenomenon, which depends solely upon the peculiar reaction of the visual nerves. In the pure domain of physics such a union never takes place objectively. Rays of different colours proceed side by side without any mutual action, and though to the eye they may appear united, they can always be separated from each other by physical means. [von Helmholtz 1852]
Helmholtz went on to demonstrate that green and red spectral colors added to make yellow, while yellow and blue added to make white. He further observed that white light could be compounded from several choices of three spectral colors. It remained to Hermann Grassmann to prove a general result in color opponency.