The human eye is an amazing structure. With your eyes, you are able to see the world around you. But vision in humans is more than just seeing the basic objects, it also allows us to perceive color. How we sense color is actually a very interesting process. The process of seeing color starts with light.
Light is made up of energy waves of varying wavelengths. Different wavelength are what makes light have a different color. So let's follow the path of a light wave in to the eye and see how it leads to a person being able to sense color.
The light enters the eye by passing through the cornea (the outer surface of the eye), through the lens (which attempts to focus the images), and on to the retina. The retina is the tissue on the back of the inner eyeball which has the receptors for sensing light of all kinds. There are two primary types of light sensors on the back of the eye. They are called rods and cones.
Rods are the light receptors which are better in low-light situations. They do not differentiate between color wavelengths. Any vision you have at night and in low light is due to the sensory receptors in the rods.
Cones are the photoreceptors that see color. They are mostly concentrated around the center of the retina, which is the part where vision is most sensitive. There are three sub-types of cones. The three types pick up long, medium, and shorter wavelengths of light. These wavelengths correspond to red, green, and blue, respectively.
When light of a correct wavelength hits a cone, the cone sends a signal to the brain that it has been stimulated. Different amounts of light at different wavelengths will trigger different numbers of cones. For example, if you look at an apple, only the red cones will be triggered. This then tells your brain that the apple is "Red".
A combination of cone stimulation will trigger a variety of the three cone types. It is then up to your brain to "interpret" these signals. For example, red and blue in equal amounts make the color that your brain thinks of as purple. Different levels of red and blue will make different shades of purple. And so there you are, you can see how different levels of cone stimulation will produce the sensation of just about any shade of color.
Cones are more sensitive in bright light. As the light entering the eye lowers in intensity, the cones become less able to discern different wavelengths. This is why everything looks gray at night. The rods are working, but the cones are not.
And that's how we sense color. Human vision is really a bit more complicated than all of this, but those are the essential concepts behind color vision.