An old adage says all paint is black until someone opens the can.

Some truth, some falsehood and some gray areas lay hidden in the adage. To find them is to understand what color is, how the eye perceives it and how we humans interpret it.

Light is electromagnetic radiation in the form of waves.

Our eyes are sensitive to a portion of that radiation with a range of wavelengths between 400 and 700 nanometers. On the average, this sensitivity matches the intensity of the solar spectrum in that range with a maximum at 550 nanometers, not surprisingly since evolution would tend to favor eyes that maximize the available light.

This spectrum is what we call white light. We define colors one way from the mnemonic learned in elementary school, ROY G BIV (red, orange, yellow, green, blue, indigo, violet).

There are other ways. Perception and division of colors vary both in individuals and in cultures.

What colors we see depends on three factors: the color of the light, the interaction of the light with objects and the interaction of light with color sensors in the retina.

Color vision stems from cones, millions of tiny sensors on the retina. Cones come in three varieties, each sensitive to reddish, greenish or bluish light.

Because of this, we can fool the eye into believing that it sees a full range of colors by combining red, green and blue lights, or cyan, magenta and yellow dyes in varying intensities. This illusion has allowed the construction of huge Jumbotron screens as well as primitive cathode ray tube color screens.

Light that reaches the eye is either emitted by light sources or reflected from surfaces. With the exception of lasers, light sources have a well-defined spectrum, referred to as color temperature.

A surface will either absorb or reflect incident light. If the surface absorbs all of the light, we see black. If it reflects all of the light, we see white.

All surfaces reflect light but absorb a proportion of the incident light so the reflected light has a different spectrum and a different color than the incident light.

For example, when white light strikes a cloth that absorbs blue and green light, it reflects red, so we see it a red cloth; is the true color of the cloth cyan based on what it absorbs, or red based on what it reflects?

On that basis, the paint in the can is black because there is no light striking it and therefore it has no reflected color.

This presents a "tree falls in the forest" conundrum: If no light is reflected and there is no one there to see it, is there no color?

The paint contains the potential to appear a certain color, depending on the light that strikes it.

We conclude that the paint is black, or any color that you want it to be, until light of some wavelength spectrum strikes it. Nevertheless, lurking in the molecular structure of the paint lies the truth, for that is what determines how the paint interacts with the light to produce the observed paint color.

Richard Brill is a professor of science at Honolulu Community College. His column runs on the first and third Fridays of the month. Email questions and comments to brill@hawaii.edu.