Most humans can see a range of wavelengths from about 400–700nm. The peak sensitivity in the human eye is around 560 nanometers, roughly the yellow color that is the primary color of sunlight.
Humans perceive light through two different structures in the retina of the eye.
Rods perceive brightness, not color. They are widely distributed in the eye, and quite sensitive.
Cones perceive color. Their density is highest near the fovea (the center of where you are looking), and they are less sensitive, functioning well only with relatively high levels of light.
Most humans have three different types of cones, each with a peak sensitivity centered around a specific frequency:
|Type of cone||Peak sensitivity|
The blue cones are somewhat less sensitive.
When considering human color perception, one must always be aware that about 5% of humans have some form of color-blindness, caused by hereditary deficiencies in the eye. Refer to the Wikipedia article on color-blindness for more details.
If you are designing a document or human interface, you can see how your color scheme looks to someone with one of the forms of color-blindness using a color-blindness simulator.