2.2 Make sure that the foreground and background colour combinations provide sufficient contrast

Pick your colour combinations with care to give a good contrast. Colour has three attributes that determine how people perceive it:

• Hue is the basic colour. For example, red, blue, green, yellow
• Lightness is tonal quality. For example, dark red. It is the most important attribute in making contrast more effective
• Saturation is the degree of colour intensity associated with a colour’s perceptual difference from a white, black or grey of equal lightness. As colours become less saturated, they move toward being either black or white

Colour schemes that have a clear differentiation in all three attributes provide the greatest contrast. An image containing light text on a dark background or dark text on a light background provides good contrast.

For text, consider these suggestions:

• Black text on white, black text on pale yellow or blue text on white are easy to read. White on black is probably OK
• Black on red and black on green are not legible, as some people will see them as black on black
• Green text on red, and red text on green are particularly hard to read for many people
• Combinations such as red-blue, green-yellow, green-white, green-grey are poor because they have small brightness contrast and because red/green colour blindness is the most common

Peoples' perception of the colour contrast varies greatly. You should never assume that if you can clearly see a colour combination as being a good contrast then others will too.

Visit the Lighthouse International for advice about colour contrasts that work.

To test whether colour contrast is sufficient to be read by people with colour deficiencies or by those with low-resolution monitors, print pages on a black and white printer (with backgrounds and colours appearing in greyscale). Photocopy the page two or three times to see how it degrades. This will show you where you need to add redundant cues (example: hyperlinks are usually underlined on Web pages), or whether the cues are too small or indistinct to hold up well.