The two theories of colour vision namely trichromtic theory (also known as Young-Helmholtz theory proposed by Thomas Young) and opponent process theory (proposed by Ewald Hering) are complementary to each other.
The opponent process theory describes the mechanism by which the information from the cones (color vision) is processed.
The retinal ganglionic cells (RGCs) are excited and inhibited by lights of different wavelengths. It results in producing two pairs of opposite colors, i.e. red versus green and blue versus yellow.