Figure 2.

Photoreceptor S captures roughly light on segments B and G, M captures light on G and Y and L on Y and R. As a result, the difference between the number of photons captured by photoreceptors L and M can be approximated by (Y + R) − (G + Y) = R − G. Likewise, the difference between the amount of light captured by photoreceptors M and S can be approximated by (G + Y) − (B + G) = Y − B (see text for details)