Most biology textbooks will tell you that photoreceptors are either rods, which are for seeing in low levels of light in black and white, while cone cells are for color vision. There are different cones for different wavelengths, in other words for sensing different colors.
New research done at the University of California, Berkeley and published in Science Advances has shown however, that not all cones are for color vision; some don't detect colors at all. The majority of them simply sent a signal of light to the brain. The work required an advanced eye-tracking device that could follow the rapid movement of the eyes. Their results came after repeated stimulation of 273 cone cells, done with two lab members. A better understanding of how these cells work may one day lead to therapeutics for color blindness and other diseases of the eye.