HUE, TONE AND SATURATION
The color appearance of a gem is the result of a combination of these three attributes. The Gemological Institute of America (GIA) system for color grading colored diamonds uses 27 hues, which are indicated on the hue circle chart. Some of these 27 hue names include modifiers, such as purplish pink.
A modifier in a hue name (such as yellowish green or orangy yellow) does not mean a lack of purity in the color.
For example - A fancy blue or a fancy greyish blue
Lightness or Darkness. The Tone of color grading refers to the lightness or darkness of the hue, or primary color of the diamond.
For color grading, colored diamonds are placed face-up in a grooved, matte-white, non-fluorescent plastic tray within a controlled environment—a viewing box that eliminates visual distractions and shields external light. GIA also requires a standard geometry between the diamond, the light source, and the observer. The light source is positioned directly above the diamond, and the observer views it approximately perpendicular to the table facet.
The GIA grading terminology uses a combination of fancy grades and color descriptions to identify a colored diamond’s characteristic color. A fancy grade represents the combined effect of tone and saturation on the color of a diamond. These grades correspond to regions of tone and saturation in color space and vary by hue, since different colors reach their highest saturation at different levels. The color descriptions accompanying a fancy grade are determined by the hue, and by the tone and saturation of the hue. In each instance, the fancy grades and color descriptions represent a range of color appearances.
Saturation refers to the dominance of hue in the color, and Intensity of it. Saturation is the measure of how strong and intense the primary color of the diamond actually is, such as light, deep, intense or vivid. A fully saturated color is the truest version of that color. Primary colors (red, yellow, and blue) are "true", so they are also fully saturated.