The main pathway of color vision is as following: light⇒photoreceptor⇒ bipolar cell ⇒Ganglion cell⇒ LGN ⇒ cortex⇒ extracortex. Here I will stress some key points of color vision.
1) photoreceptors ( mainly cones, because rods are sensitive to low light level and blind to color) absorb light, and through photochemical reactions transfer solar energy to electrical energy which can be indirectly displayed as action potential of cells. There are three cones: L, M, S cones respectively. L cones are responsible to long wavelength; M for middle wavelength; and S for short wavelength. The typical receptive field of cones is center and surround pattern. For example, on-center Receptive Field will maximally activate if the surrounding was dark and center was light-on; the reverse is true for the off-center RF. There are two complementary theories of color vision: One of them is the dominant and popular trichromatic theory, which argues that three types of cones (L, M, S) are sufficient to code the color vision. However, Hering raised a different theory--- color opponent theory. He argued that there were four elemental colors (R,Y, G, and B) not three. He also noted the pairing of R & G, and of B & Y. Three cone types provides input to two different “color opponent” ganglion cells. For example, the opponent neuron that produces the neural response we see as red or green gets excitation from the L cones and inhibition from the M cones. When the L cones are more active than the M cones (as would happen with a long wavelength stimulus), the net result is excitation, and we see “red”. If, however, the M cones were more active (if stimulated with a 500 nm wavelength), the net result would be inhibition, and we would see “green”.
2) LGN: There are three pathways in the LGN level: M-Pathway of LGN, that is, the L-cones and M-cones send signal ⇒diffuse Bipolar Cells ⇒ M-ganglion cells⇒ M-pathway. P-Pathway of LGN, that is, the L-cones or M-cones send signal ⇒midget Bipolar Cells ⇒ P-ganglion cells⇒ P-pathway. The last pathway is K-pathway: S-cones send signal ⇒ blue-cone Bipolar Cells ⇒ K-ganglion cells⇒ K-pathway.
3) Primary visual Cortex (V1): In V1, the M-pathway, P-Pathway, and K-pathway will end up at different layers. For example, M-Pathway will end up at layer 4C(alpha) and 6; the P-pathway will end up at layer 4C(beta), 4A, and 6; and K-pathway will end up at layer 2 and 3, where there are rich of CO, and some end up at layer 1.
4) Extrastriate cortex (V2, V3, V4, V5, MT MST eta): In V2, the M-pathway, P-Pathway, and K-pathway will end up at CO-rich bands: For example, the M-pathway will overlap with the thick CO-rich bands; the P-pathway will overlap with the thin CO-rich bands; the K-pathway will overlap with the thin CO-rich bands. Beyond V2, there are two separate pathways to process color: One is the ventral pathway; the ventral pathway is made up primary of V4 and some V2. The other is the dorsal pathway, which is composed of V3, MT, MST....