The Quantitative Contrast

The quantitative contrast can be explained very easily: There is a big area of any color juxtaposed to a small area of another color. The size of the areas depends on the situation. As a guideline, the small area should not take up more than 20% of the big area. It is possible that the small area takes up only 1% or even less, but the color should still be clearly visible.
 
Intense colors can appear very luminous on computer screens. Extensive use of those colors causes a shock effect that jars your eye. The bigger area on a web site should therefore be brightened with white or darkened with black or the color's intensity should be decreased by mixing it with gray. The smaller area should then be of a more colorful, more intense and brighter color. If you kept it the other way round, the small area would not stand out at all, because it is "outshined" by the bigger area.