Yellow = Green + Red

To help understand this reduction, consider the perception of ``yellow''. According to the visible light spectrum (Figure 4.5), yellow has a wavelength of about $580$nm. Suppose we had a pure light source that shines light of exactly $580$nm wavelength onto our retinas with no other wavelengths. The spectral distribution function would have a spike at $580$nm and be zero everywhere else. If we had a cone with peak detection at $580$nm and no sensitivity to other wavelengths, then it would perfectly detect yellow. Instead, we perceive yellow by activation of both green and red cones because their sensitivity regions (Figure 5.3) include $580$nm. It should then be possible to generate the same photoreceptor response by sending a jumble of light that contains precisely two wavelengths: 1) Some ``green'' at $533$nm, and 2) some ``red'' at $564$nm. If the magnitudes of green and red are tuned so that the green and red cones activate in the same way as they did for pure yellow, then it becomes impossible for our visual system to distinguish the green/red mixture from pure yellow. Both are perceived as ``yellow''. This matching of colors from red, green and blue components is called metamerism. Such a blending is precisely what is done on a RGB display to produce yellow. Suppose the intensity of each color ranges from 0 (dark) to $255$ (bright). Red is produced by RGB $=(255,0,0)$, and green is RGB $=(0,255,0)$. These each activate one LED (or LCD) color, thereby producing a pure red or green. If both are turned on, then yellow is perceived. Thus, yellow is RGB $=(255,255,0)$.