The lemon itself has no color; your brain constructs this perception of color in two stages. First, the lemon reflects light energy into your eyes, where it is transformed into neural messages. Three sets of cones, each sensitive to a different light frequency (red, blue, and green) process color. In this case, the light energy stimulates both red-sensitive and greensensitive cones. In the second stage, a network of opponent-process cells sensitive to paired opposites of color (red/green, black/white, and yellow/blue) evaluates the incoming neural messages as they pass through your optic nerve to the thalamus and visual cortex. When the yellow-sensitive opponent-process cells are stimulated, you identify the lemon as yellow.