The applet (left) draws a slice through the RGB colour cube at a particular colour level = red + green + blue.

The mouse can be used to change the colour level - click near the top of the applet to raise the level, click near the bottom of the applet to lower the level.

Using the applet (above) we are looking down the diagonal of the cube from white towards black.

The slices through the cube are triangular near to black (#000000) or white (#FFFFFF) and hexagonal when the overall level is between 255 (#FF) and 510 (#1FE)