Lightness or Darkness. The Tone of colour grading refers to the lightness or darkness of the hue, or primary colour of the diamond.
Saturation refers to the dominance of hue in the colour, and Intensity of it. Saturation is the measure of how strong and intense the primary colour of the diamond actually is, such as light, deep, intense or vivid. A fully saturated colour is the truest version of that colour. Primary colours (red, yellow, and blue) are “true”, so they are also fully saturated.

There are six categories in the GIA scale that equate to colour saturation.