The red, green and blue are used as the three primary colors which span a 3D space. The intensities of these primaries are calibrated to range from 0 to 1 so that all possible colors that can be produced by these primaries are located inside the cube as shown:
The origin of the coordinate system is black where 
, while the point
represents white. The line segment formed by the black and white
points is the line of grays representing different gray levels. A cross section
plane perpendicular to the line of grays and going through point 
is shown in the figure
As can be seen this cross section has the shape of a hexagon with highly saturated hues along its sides (e.g., red, yellow, green, cyan, blue, and magenta in the middle part of the six sides, respectively), Also we see that the closer a color is to the line of grays, the less saturated it becomes. The third variable intensity is not represented in this cross section.
The hue, saturation and intensity of colors can also be represented by a similar model as shown below:
A horizontal cross section going through the middle of the cone is shown here:
Two perpendicular vertical cross sections cutting through center of the cone are also shown:
