It's a question Aristotle could not have begun to answer.

Not until Newton did anyone have a clue about where to start.  Newton discovered the spectrum, and that led to some understanding that light resembles waves in water.  (Exactly how is still a bit of a mystery at the edge of human comprehension because the resemblance is more metaphor than model.)

Color is information about the wavelength of the light we see.  Or if you wish, the frequency, which is another way of describing much the same thing.

But it doesn't tell all.

If fact, it tells very little.

You see a piece of white paper.  Why is it white?  What is white light?
A common answer is that light is white when it's an equal mixture of all three "primary" colors.  Alternatively we are told that white is a mixture of all colors in equal proportions.  A piece of paper is white if it reflects all colors equally.  These are not very good explanations...a little bit right; a little bit wrong.

    light that gets a roughly equal response from each of the three families of cones on our retinas.  The computer screen you are looking at was constructed with this feature of the human eye in mind.  Look at it with a strong magnifying glass and you will see an array of colored dots: equal numbers of blue dots, green dots, and red dots.  Where the screen is white, the blue dots, the green dots, and the red dots are all at maximum brightness.  There are no white dots. 

We cannot have experiential knowledge of bird color.  We are blind to that bird-brain knowledge.  It's beyond the edges of our comprehension.  However, we can, through mathematics, ask and answer meaningful questions about what a bird sees.  The answers we get cannot relate directly to human experience, and we can be sure that those answers will be "simple but difficult."

Much of science is like that: an attempt to peer beyond our evolution-developed limits.
 
 

To understand what  white light is we must go beyond what we learned in grade school.  It's not that white light is simply equal intensities of three "primary colors," or of all wavelengths, or even of all visible wavelengths.

Because:

• Infrared and longer wavelengths might be present, but our perception isn't affected by those wavelengths which we can't see. 
• Ultraviolet and shorter wavelengths might be present, and our eyes don't see those either, but ultraviolet can, nevertheless, effect what we see in several ways.  Ultraviolet on our eyes can cause the corneas and lenses to fluoresce and put a haze on the images we see.  Ultraviolet causes many other things to fluoresce, such as those bright orange detergent boxes in the supermarket.  More orange light comes away from the box than falls on the box.  That's to get your attention.  "Whiter than white" is not just advertising hype: If the detergent leaves a fluorescent substance in the laundry, the whites will reflect more visible light than falls on them: they really are whiter than white.
• We can get equal responses from each of the three cones with light that has virtually all of the visible wavelengths removed.  There are an infinite number of visible wavelengths–light is a continuum [catch-22] of wavelengths.  Three wavelengths from that infinity are all that's needed for us to perceive white, and those three do not have to be those we learn in grade school as "primary" colors.

These are not facts that fit those lessons from the third grade.  These facts do show that color is not simply a characteristic of the world around us, but also a trait of the way we evolved to perceive and interact with that world.

Color can be better understood if we learn about one very special solution to that puzzle of arranging colors so that proximity and similarity correlate.

•    Throughout these Web pages you will see many unusual uses of color.  Are maps hard to read?  So is the page on which the problem is introduced: there, we used color to demonstrate one kind of difficulty of reading.  Does the thought of drowning in quicksand jar your senses?  So does the title of the quicksand page: there, we demonstrate the difference of focusing planes for different colors to emphasize the point.  But the really unusual (perhaps it's even unique!) use of color is the demonstration of one of the most pernicious and pervasive oversimplifications you will see:  the "scalar-reduction" of virtually all measures.  It's one of the symptoms of Herpes simpletonisus.