What is the purpose of this cube in color quantization? So, I go to install GMIC for Gimp via the package manager and landed myself upon the demo program. I have discovered things such as Hough transform and  color quantization. So, I get the number of colors on the bottom bar, I understand what it is doing, but I do not get the cube in the middle, mainly why it's 3D or that it has other shapes inside of it (other than the dots for the colors chosen). Basically, I want to know the purpose of this cube, such as why is it a cube, or why is it 3D.



Edit: 

I hope this is a better picture. Turns out the image is different too!
The cube looks like just an RGB cube, i.e. X,Y,Z map to R,G,B. Since there are 3 color channels, the 3D cube is a convenient way to visualize the space of all possible RGB colors. However, I don't know what the pie-wedge-looking shape outlined inside the cube is.