All paint inside a can is black until the can is opened. Strange as it might seem, it is true. What we see as color is a matter of perception, intrinsic properties of an object and the light that illuminates it.

Because there is no light inside the paint can while the lid is on, the paint is black. Pure black is the absence of light. Turn out the light in the closet with the door closed and everything appears black.

White light is a horse of a different color. There is no such color as “white.” White light is a perception of a combination of all colors of the spectrum, but not all “white” spectra are the same. The spectrum of all light that we identify as white is brightest in the middle and drops off in intensity in both directions toward red and blue.

The human visual system quickly adapts to and corrects for changes in the spectrum of light so that we perceive the same object as white in sunlight or on a cloudy day. Many different spectra can seem white to us, so it is impossible to define what we perceive as “white” objectively.

The spectrum of sunlight has maximum intensity in the green region around 550 nanometers, and this is traditionally taken as the standard “white.”

Sunlight, an incandescent bulb and a fluorescent light each consist of a spectrum with a different maximum intensity. The spectrum is categorized by its color temperature, which is expressed in Kelvins, a close approximation to Celsius at high temperatures.

A substance at a temperature of 6,000 K is at the same temperature as the surface of the sun and emits light that has a spectrum identical to that of the sun.

Modern LEDs have their spectrum adjusted to conform to either daylight or warm white.

The tungsten filament of a 40-watt incandescent bulb is at a temperature around 2,500 K. At this temperature its peak is in the infrared, and it emits energy as heat while glowing with a yellow tint.

Most objects are visible only because they reflect or transmit light; they absorb selected wavelengths from the spectrum. The reflected or transmitted light contains the remaining wavelengths that we perceive as the color of the object.

An orange is orange in white light because it absorbs its complementary color of azure, a slightly greenish blue, and reflects light in the range of 585-620 nanometers. In blue light an orange is black, so the perceived color of the orange depends on the color of the light that illuminates it.

Resonance at the atomic level determines what colors are absorbed, transmitted or reflected by an object or a substance.

Electrons that form the bonds holding the atoms of solids and liquids together have resonant frequencies that are unique for a specific substance.

If those frequencies are the same as the frequency of light that strikes it, the electrons resonate and gain energy, which manifests as reflected light. Light of wavelengths that are not in the range of the resonant frequencies of the substance are absorbed.

There are exceptions, as in the colors imparted to glass and gemstones by transition metals such as iron, nickel and chromium, but that will have to wait for another column.

Look around at the variety of colors and the way the colors change on a sunny day versus a cloudy day or between sunrise/sunset and midday. It is awesome that vibrating electrons that are too small to see influence our mood and our innate sense of time by coloring our world.

Richard Brill is a retired 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.