Is color real? Does everyone see the same color green? How many colors can the human eye see? Color helps us to remember objects, influences our purchases and affects our emotions. (Are you blue?) More amazing is that objects without light do not have color! Lemons are not yellow but it is how their pigments reflect the wavelengths of light into the human brain that they appear yellow.
Reena Mukamal explains in How Humans See In Color that human brains see color between ultraviolet light and red light. Scientists estimate that humans can distinguish up to 10 million colors. When light hits an object, such as lemon, the object absorbs some of the light and reflects the rest of it. That reflected light enters the human eye through the cornea which bends the light toward the pupil. The pupil controls the amount of light that hits the lens. The lens focuses the light on the retina which is a layer of nerve cells in the back of the eye.
The retina has two different types of cells, rods and cones, which detect and respond to light. The cells that are sensitive to light are called photoreceptors. Rods are activated in dim light and cones in a brighter environment. Most humans have about 6 million cones and 110 million rods.
The cones contain “color-detecting molecules”. Typically there are three types of these molecules, red, green, blue. Each type of cone (red, green or blue) is sensitive to different wavelengths of visible light.
In the daytime, a lemon’s reflected light activates both red and green cones. The cones then send a signal along the optic nerve to the visual cortex of the brain. The brain processes the number of cones that were activate and the strength of their signal. Alter the nerve impulses are processed, a color is seen. In the case of the lemon, it is yellow.
In a darker environment, the light reflected by the lemon would stimulate only the eyes’ rods. If only the rods are activated, you don’t see color, just shades of gray. Another perception of color is “color constancy”. If you have seen the lemon in several different conditions, for instance red light or dim light, you would still perceive the lemon to be yellow.
Color blindness is a problem with the cones. They can be absent, nonfunctioning or detect a different color than normal. Red-green color blindness is the most common followed by blue-yellow and it is more common in males.
Researchers estimate that up to 12 % of females have four cone types. They would then have the potential to perceive 100 times more colors.
Visual systems vary from person to person. An object’s color may appear differently against different backgrounds or under different lighting. There is a surprising amount of variation in how people see the world but the colors of a sunset remain beautiful. At least humans could agree on that beauty even if every sunset offers different colors.
by Sally Robinson, MD Clinical Professor
Keeping Kids Healthy
Published 05/2025