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The color of a stamp is described with a combination of
color names, the names coming from a fixed palette.
For a type with more than one color, the color is expressed as "name1&name2&...". While theoretically unlimited, after about four color components, it is convenient simply to use the special term "multicolored" or "multi" for short; this is also appropriate for any sort of full-color printing process. Some stamps have a full-color design embedded in a single-color background, which can be described as "color&multi".
For stamps with two colors, the assumption is generally that the color of the frame or outer area comes first, followed by the color of the vignette. (This follows Scott, but is exactly opposite to the Stanley Gibbons practice; beware!)
The names of the possible colors are all predefined, and are all listed on the color guide page. They fall into several forms:
Most names and modifiers have abbreviations, so "light yellow green" shortens to "ltyelgrn".
(Although color names instead of numeric values may seem old-fashioned for the computer age, actual stamps may vary considerably in color, depending on heaviness of inking, fading due to age, etc. Names avoid a false precision, and are easier to remember.)