Hue

Hue, in its most basic meaning, is the classification of the rainbow color. More specifically, it is the dominant wavelength of color depicted by the name such as red, blue, or green. These colors range from 0-360; 0 = Red, 120 = green, blue = 240, etc. Each individual hue is determined and can be further classified by its chroma, saturation, intensity, value, and shade. The hue is invariant under changes of direction and intensity of the incident light. The hue describes the dominant color ingredient that one perceives. For this reason, if we preserve the hue and enhance the brightness, the obtained image will appear more colorful. Hues have a family of twelve purest and brightest colors; three primary colors, three secondary colors and six tertiary colors, which form a full spectrum of colors that progress around the primary color wheel. Hue is one of six color appearance parameters, the other four are chroma, saturation, lightness, brightness and colorfulness.

Hue is especially important in the area of cartography where it is often used to distinguish between different elements on a map. Certain conventions exist for the use of some hues in cartographic design, such as blue for water or green for vegetation; such conventions have been used for many centuries. Hue is especially useful in displaying nominal, or qualitative data, where different hues represent different, unrelated phenomena. Hue is especially useful in displaying nominal, or qualitative data, where different hues represent different, unrelated phenomena. For example, a point layer representing cities in the United States could be divided into two groups: green points representing the capital cities and blue points representing all other cities. The two types of data are on the same map, but do not have the same meaning. In this case it makes sense to use two different hues (blue and green) to differentiate the nominal data. Whereas, when working with quantitative data, hue is best used in combination with value or  saturation.

Active colors
Active colors are brighter colors of hue such as red, yellow and orange. They tend to described as warm and give feelings of confidence and enthusiasm.

Passive colors
Passive colors are the more neutral hue colors such as blue, purple, and green. The feelings generally associated with passive colors are calming and focusing.