Two shades of gray along the black-white axis: White light is made up of the three primary colors--red, green, and blue--mixed in equal proportion at full intensity. As you decrease intensity, you go through all the shades of gray until you reach black.

The secondary colors--Yellow, Cyan (light blue), and Magenta (light purple)--are a combination of two primary colors in equal proportion.

When you mix two primary colors in different proportions, you can generate a continuum from one to the other, a portion of the color wheel. It goes through the secondary color formed by those two primary colors.

This zig-zag wheel of color goes from corner to corner of the color cube and circles an axis of the shades of gray that goes through Black (bottom, front, left) and While (top, back, right).

There is an Outer Wheel (above, top row) and an Inner Wheel (above, bottom row).

Imagine the colors placed at the corners of a cube. The path from Black (bottom, front, left) to each of the primary colors goes through the shades of those three colors. The path from White (top, back, right) to each of the secondary colors goes through the tints of those three colors. These paths go along the edges of the cube.

Diagonal lines (not shown in the diagram above) that go across the faces of the cube connect (a) Black with Secondary colors and (b) White with primary colors.

Unequal mixures of a primary and a secondary color on the Outer Wheel (below, center columns) lie on the edge that connects them.

Equal mixures of a primary and a secondary color with shading (above, left column) or tinting (above, right column) lie on the faces of the cube.

We can characterize any color as a combination of four: white (tinting), one of the secondary colors (made up of two primary colors), a primary color (one of those two colors), and black (shading).

Only combinations of three colors, however, fit into the 64-color matrix. With only four increments (00-55-aa-ff) in each dimension, there are only three slots for the colors.