focus on low vision

Effective Color Contrast

by Aries Arditi, PhD

How do you design for people with partial sight and color deficiencies? This article contains three basic guidelines for making effective color choices that work for nearly everyone. Following them are explanations of the three perceptual attributes of color—hue, lightness, and saturation—as they are used by vision scientists.

How does impaired vision affect color perception?

Partial sight, aging, and congenital color deficits all produce changes in perception that reduce the visual effectiveness of certain color combinations. Two colors that contrast sharply to someone with normal vision may be far less distinguishable to someone with a visual disorder.

CONTRAST IS KEY

It is important to appreciate that it is the contrast of colors one against another that makes them more or less discernible rather than the individual colors themselves. Here are three simple rules for making effective color choices:

1 LIGHTNESS DIFFERENCES. Exaggerate lightness differences between foreground and background colors. Avoid using colors of similar lightness adjacent to one another, even if they differ in saturation or hue.

Don't assume that the lightness you perceive will be the same as the lightness perceived by people with color deficits. You can generally assume that they will see less contrast between colors than you will. If you lighten the light colors and darken the dark colors in your design, you will increase its visual accessibility.

2 DARK AND LIGHT. Choose dark colors with hues from the bottom half of the hue circle (see Color Hue circle at the top of the opposite page) against light colors from the top half of the circle. Avoid contrasting light colors from the bottom half against dark colors from the top half.

For most people with partial sight and/or congenital color deficiencies, the lightness values of colors in the bottom half of the hue circle tend to be reduced.

3 ADJACENT HUES. Avoid contrasting hues from adjacent parts of the hue circle, especially if the colors do not contrast sharply in lightness.

Color deficiencies associated with partial sight and congenital deficiencies make it difficult to discriminate between colors of similar hue.

PERCEPTUAL ATTRIBUTES

Hue, lightness, and saturation are the three perceptual attributes of color. Hue varies around the solid; lightness varies from top to bottom; and saturation is the distance from the center (see Perceptual Attributes image).

• HUE is the perceptual attribute associated with elementary color names. It enables us to identify basic color categories such as blue, green, yellow, red, and purple.

People with normal color vision report that hues follow a natural sequence based on their similarity to one another. With most color deficits, the ability to discriminate between colors on the basis of hue is diminished.

COLOR HUES: Choose dark colors with hues from the bottom half of the hue circle against light colors from the top half of the circle

• LIGHTNESS corresponds to how much light appears to be reflected from a colored surface in relation to nearby surfaces.

Lightness, like hue, is a perceptual attribute that cannot be computed from physical measurements alone. It is the most important attribute in making contrast more effective.

PERCEPTUAL ATTRIBUTES: Hue varies around the solid; lightness varies from top to bottom; and saturation is the distance from the center

• SATURATION is an issue as well. With color deficits, the ability to discriminate colors on the basis of lightness is reduced.

PANEL VIEWS: To a person with color-deficient partial sight, the left-hand panel might appear like the right-hand panel appears to a person with normal color vision

As shown in the Panel Views illustration, at left, to a person with color-deficient partial sight, the left-hand panel might appear like the right-hand panel appears to a person with normal color vision.

With color deficits, the ability to discriminate colors on the basis of all three attributes—hue, lightness, and saturation—is reduced.

Designers [or anyone helping create a living environment for a partially sighted or color-deficient person] can help to compensate for these deficits by making colors differ more dramatically in all three attributes. EB

Aries Arditi, PhD, is Senior Fellow in Vision Science, Lighthouse International. This article is based on his earlier work with Kenneth Knoblauch. To view a copy of Lighthouse International's copyrighted article upon which this column is based, visit the organization's website at www.lighthouse.org/color_contrast.htm.