Color perception is a fascinating series of physical and chemical reactions which allow some organisms to see in color. The process of color perception is literally all in the mind, with the eye containing the equipment which responds to light so that the brain can process it. The number of colors an organism can distinguish can vary considerably, from animals that can see a very wide array of millions of colors to animals that see in a much more limited range.
Two types of cells in the eye are responsible for vision: rods and cones. Both cells are located in the retina, and they respond to light when it enters the eye. Rods are highly light-sensitive, allowing for vision in a range of light levels, while cones are sensitized to colors of particular wavelength ranges. Humans have three different types of cones sensitized to short, medium, and long wavelengths, and they are especially sensitive to yellow and green light. Organisms with three types of cones are known as trichromatic, and other animals may have two types of cones (dichromatic), while others have up to five (pentachromatic).
Until 2005, researchers assumed that the number of cones in the eye was roughly the same for all people, and that all people had the same numbers of different types of cones. However, when imaging technology which could be used to look into a living eye and distinguish between the different types of cones present was developed, researchers learned that people actually have very irregular distributions of cones. 40% of one person's cones might be sensitive to medium wavelength light, while someone else might only have half that number, but both people would perceive color in the same way, which strongly suggests that the key aspects of color perception happen in the brain, not the eye.
When light enters the eye and hits the cones, certain cones fire in response to the wavelength they are sensitized to. The response is sent along the optic nerve to the brain, and the brain processes the information, allowing the organism to see in color. Color perception is important for many animals because it can be used to distinguish more of the natural environment, and because colors are often used as cues. Poisonous mushrooms, for example, are sometimes brightly colored as a warning.
The process of visual perception happens so quickly that people feel like it is instant, as they look at something and immediately see it. The eyes also constantly return new information, allowing people to detect changes such as movement. Although the basic mechanics of how vision and color perception work are understood, the processes are still a little bit mysterious to researchers. The mechanisms behind color blindness, for example, are not fully understood.