How does opponent-process theory explain why reddish green and bluish yellow are not perceived?

The opponent-process theory, first proposed by Ewald Hering in 1878, posits that human colour vision is governed by three distinct mechanisms, each responding to a pair of opposing colours: light–dark, red–green, and blue–yellow.1

This theory explains why we do not perceive "reddish green" or "bluish yellow" because these colours are processed as mutually exclusive opposites within the same neural mechanism. In this framework, the visual system functions through opponent channels where the activation of one colour in a pair inhibits the perception of its counterpart. For example, neural cells in the visual pathway are organized such that they produce chromatic signals for blue–yellow and green–red; because these signals are opponent, they cannot coexist in a single perceived colour.12

Modern understanding integrates this with the trichromatic theory through "zone theories," which suggest that while the eye’s cones function trichromatically, their signals are later combined in neural cells to create these opponent chromatic signals that the brain interprets.1