Cones
In order for color to be perceived, it is necessary to have different photoreceptors that respond differently to the frequencies of light. These are the cones. The chromophore molecule retinal is identical in each variety of cone, so it is the opsin protein that is responsible for the differential frequency response.
The energy of a photon is directly proportional to its frequency. Higher frequencies, hence higher energy photons, are on the blue side of the visual spectrum. Conversely lower frequencies, on the red side of the spectrum, have lower energies.
It requires a specific energy for an electron to absorb a photon, since it must be the right amount needed to push it into the next higher orbital.
More detail
Opsins, like all proteins, are chains of amino acids like a string of beads. DNA contains the codes that determine which of the 20 amino acids are in each position in the chain. But a long chain of amino acids in a simple string would be able to do very little. Proteins gain their function by folding into complex shapes. The folding is determined by the shape and electrical charge of each amino acid, as well as by whether the amino acid is more comfortable in a water or an oily environment. A common shape for segments of a protein is a helix that is similar to a cylinder and has a somewhat rigid structure like a bar or a rod.
Opsins are proteins containing about 350 amino acids and are located in the photoreceptor cell membrane. Opsins have 7 such cylindrical helixes spanning the membrane. Retinal is a small molecule attached to the amino acid lysine on helix number 7.
Only one end of retinal is attached to its opsin. The free end is held in a “pocket” of the opsin formed by the amino acids nearby. The bent retinal fits the pocket, but is under tension caused by the attraction and repulsion of the nearby electrical charges. This tension wants to twist the retinal at the bending spot.
The electron pair in the perpendicular orbital at this bending spot is influenced by the electrons in nearby atoms, affecting the amount of energy needed to push one up to the next orbital. The quantum nature of this energy is responsible for the sharp peak in the photopigment frequency response curve. In this way, the opsin protein is able to tune the frequency response of the retinal. This is responsible for the variation in responses of the different types of cones.
The elongation of retinal pushes apart some of the opsin helixes, opening up a cleft for the attachment of a signal molecule called a G protein. The G protein activates an enzyme that opens the cGMP ring, shutting the gate on the sodium channel. This increases the negative charge inside the cell (hyperpolarizing it), which triggers an action potential being sent down the axon of the rod or cone.