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 at 
representing the black
color, while the point 
represents the white color. The line segment
between 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
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:
