U n d e r s t a n d i n g
C o l or

What we see is not always what is out there.

What you see is NOT just determined by the light signal. It depends on:

the actual light signal entering the eye
processing of this light in the eye
multiple levels of processing in the brain
the sum total of all your experiences in the world

Light enters in the eye, where it excites the rods and cones. Here, much of the information is averaged/enhanced and/or discarded. For example:

Edges are enhanced (Mach Bands)
color-blind people lose some color information that color-seeing people do not.

The resulting signal from the eye is sent along the optic nerve to the primary visual cortex in the brain. What happens in the visual cortex is greatly effected by your experiences in the world. For example, most people's experience in the world result in:

Color constancy - an object seems to be the same color regardless of the lighting conditions.
We assume that objects farther away will be smaller.
Are the dots out or in? In the picture below, there is ambiguity - is the light source from above or below? What is the orientation of the box? How does your brain resolve these ambiguities?
What color is it?

Note, blindness can be due either to damage to the eye/optic nerve or due to damage to the visual cortex (blindsight).

Identifying Objects

Contrast provides a way of creating a focus in your artwork. This contrast can be through

color/texture
shape/line
level of detail
etc.

Our ability to identify differences (e.g. of color or shape) varies. For example, in the images below, how easy/hard is it to identify if there is a red circle in the image? The difficulty depends on the objects around it. What do you think?

Color Perception

Human eye can distinquish hundreds of thousandsof colors based on humans comparing colors side by side. When judging colors based only on hue - there are about 128 distinguishable saturated hues, 130 tints (saturations) and for each or these, a number of shades depending on the hue.

Spectra Map to a Single Perceived Color

The combination of wavelengths present determine what we perceive as the color of the object.
If 700nm is the predominant wavelength, then we perceive the color as red. We say red is the dominant wavelength.

Color spectrums are rarely this simple. However, whatever the spectrum, we always perceive a single color. Many different spectra can map to the same color. Spectra that map to the same perception of color are called metamers. Metamerism is the basis of all color reproduction technologies.

In many cases, it can map to a single frequency. Magenta, which is a combination of red and violet is one example of a color that is not found in the frequency spectrum.

Common Terminology

Other characteristics of the light perception are

the brightness or intensity of the light. Intensity is the radiant energy of the light emitted per unit time per unit solid angle and per unit projected area of source. In the graph on the below, brightness corresponds to the area under curve. Radiant energy is related to the luminance.
the purity of saturation describes how washed out the color looks. In the graph below, purity corresponds to the difference between ED and Ew. The greater the difference, the greater the purity.

Note, the area under curves that differ only by saturation is constant as shown in the graph below.
Hue is what we think of as color.
Chromaticity refers to purity and dominant frequency. (and not luminance)