The subtitle for this page should probably be, “do you remember Physics 101?” Color would not be possible without light; in fact, color is light—either viewed directly from a light source or reflected from objects that we see. What humans perceive as visible light occupies a tiny portion of the electromagnetic spectrum, from 4.3–7.5×1014 Hz in frequency (700 nm to 400 nm in wavelength). Below visible light is infrared, microwave, radio and TV, and electric power. Above it is ultraviolet, X-rays, and gamma rays. The frequency of the radiation determines the color of the light.
• The lowest visible frequency is called red; the highest is violet.
• White light is an even distribution of all visible frequencies. Rainbows and prisms divide white light into the colors of the spectrum.
• What we call black is simply the absence of light.
A white surface reflects all light frequencies equally; a black surface reflects no light. Colored surfaces reflect some frequencies and absorb others.
Absolutely pure white light, as defined above, occurs only in laboratories; pure (single-wavelength) colors occur only in lasers; absolute black (no light) occurs only in photographic darkrooms. The same lack of precision is true of reflective surfaces, although our brains have been trained to distinguish colors under the normal lighting conditions that we encounter in daily life.
Even people with full "normal" color vision do not perceive all of the colors of the spectrum equally well. Our greatest sensitivity is approximately centered around green; in effect, we can distinguish more subtle shades of color between yellow and cyan than we can at either end. Our sensitivity at the bottom end (red) is much lower, but still somewhat better than at the high end (blue).