As many people know, "white" light from the sun can be diffracted or split through a prism to yield a variety of colours. In the real world,  this phenomena is apparent when we see rainbows. In order for our eyes to perceive light, in general, we have two different receptors in the back of our eye: cones and rods. Cones and rods have complementary abilities. Rods can work with a very fast response rate, and are incredibly sensitive at low light levels. However, they are generally insensitive to colours. As such, rods form the "black and white" or "contrast" portion of what we see. Conversely, cones are the "colour" viewing portion of our eyes. Cones respond to particular wavelengths of light, and give us a highly detailed coloured view of the world. In order to function though, cones need more light intensity than rods, so they are limited in low light levels. In this way, cones tend to be like the "brightness" meter of our eyes.

In cones, there are pigments that respond to three colours: blue, green and red. Each of these pigments gets "excited" by a different wavelengths of light to varying degrees. When each pigments is excited, it sends a message to your brain, which then matches all these little coloured responses to make up the coloured world that we see. Here is a sample of what the pigment responds to. Each of these simplified curves shows the absorbance of each pigments; the actual absorbance curves are considerable more complex. The higher the absorbance, the more excitations occur, and the intensity of that colour (as we observe it) increases.

As you can see, there are pigments that absorb heavily at low wavelengths (blue light), medium wavelengths (green light) and higher wavelengths (red light).  Altogether, these form the limits of light that out eyes can see and interpret. At lower wavelengths, there is high energy UV light, and at very long wavelengths there is low energy infared radiation.

An interesting thing to note is that our eyes do not have equal amounts of each of these three pigments. It turns out that the average human will have more red and green active pigments than blue responsive pigments. This happens to be a functional and natural adaptation; it arises from the fact that our Sun tends to output a lot of light in the "yellow" range due to the presence of hydrogen in the Sun. Another thing that is quite interesting is that there is considerable overlap in the pigments. For example, just under 500 nm, there is still considerable absorbance by both the blue and the green pigments. Similarly, around 620 nm, there is considerable overlap between the green and red pigments. It turns out that our eye determines the colour we see by mixing together these three primary shades of colour.