Colour is commonly understood as having 3 dimensions: Hue, Chroma and Value.
Hue is what we commonly think of as colour; for example, different combinations of Red, Green and Blue.
Value, or lightness, varies vertically along the color solid, from black at the bottom, to white at the top. Neutral grays lie along the vertical axis between black and white.
Chroma, measured radially from the center of each slice, represents the saturation of a colour, with lower chroma being less saturated (more washed out, as in pastels), and higher chroma being more saturated.