Despite all the mathematical certainty inherent in the physics of light waves, color is ultimately a strongly subjective perception. No two people actually "see" the same color because the variables that affect our perception of color differ from person to person. Ultimately, it can be said that color only exists in the mind of the person viewing it.

These variables fall into three main categories: pyschological/emotional, physiological, and environmental.

Psychological/Emotional Variables
People have emotional responses to color. For example, red is regarded as a "hot" color that provokes aggression—we often use the term "seeing red" when someone is mad—while blues are cool and relaxing. We associate black with death, but black also evokes a sense of dignity. We associate white with purity and green with fertility. Being "in the red" is bad financial news while being "in the pink" means you're healthy.

Red cars may unduly attract attention, while more sedately colored vehicles often escape notice. Apart from blueberries and blue M&Ms, there is no blue food. Blue is regarded as an appetite suppressant (other than blueberries, blue does not occur naturally in any food item).

Psychological and emotional factors in color perception also play a big role in defining color. What two or more people see as a pure expression of a given color may differ to a great degree. For example, we might define "yellow" using any of the following formulae:

H: 60 S: 100 L: 100

R: 255 G: 255 B: 0

L: 97 a: -20 b: 88

C: 0 Y: 100 M: 0 K: 0

But looking at the colors below, which would you say is more purely "yellow" than the others?

Is the one that most closely fits a formula, or is it the one that most closely matches what you think yellow should be?

This is a significant issue in color-critical fields like advertising since people respond to a product, or to the advertising of that product, by how they feel about the colors associated with it.

Physiological Variables
As we outlined in the preceding section, your eyes have a lot to do with the perception of color. The color-sensitive pigments in the cones of your eyes determine what signals are sent to your brain, giving you the sensation of color. But what if those pigments are deficient?

One in 30 people are colorblind to some extent. This is mostly a deficiency in seeing reds/greens or yellows/blues, but may also be a complete inability to distinguish colors.

Environmental Variables
The major environmental variable concerns the kind of ambient light under which a color is seen. This is directly related to the spectral power distributions discussed earlier. What we see outdoors is illuminated by the sun which is a, roughly, 6000K source. Light from artificial sources is rarely that bright. Since luminance is an important factor in seeing color, the brightness of your environment will have a lot to do with the color you see.

Metamerism
Another aspect of lighting is the issue of metamerism. Two objects may appear to be identical colors under a certain kind of light, yet under another kind of light they may appear distinctly different:

metamerism is caused by two colored objects with differing spectral reflections (known as metamers) but which, under light of a particular spectral power distribution, provoke the same stimulus in the r, g, and b cones of the human eye.

Achromatic Simultaneous Contrast
Another environmental variable is what is called achromatic simultaneous contrast. This is the juxtaposition of differing colors that affect each other. This can be seen in the illustration below:

The red dots in each quarter have the same spectral reflectance; however, due to the colors in each quarter that surround them, the dots appear darker or lighter than each other. The stimulus to the eye of the surrounding color affects the stimulus of the red dot.