Rhodopsin works not because they detect colour but because when light hits the rhodopsin it causes a chemical change which causes it to decompose. This decomposition changes the rhodopsin molecule to active rhodopsin and the active rhodopsin changes the charge of the rod cell. This creates an electric current along the cell which is sent to the ganglion which is connected to the optic nerve. The more light the more electric current. The optic nerve will then send the message to the visual cortex of the brain where the light will be interpreted as an image.