White is at the opposite corner of the cube from black because it is made by adding an equal maximum amount of all three primaries. Along the diagonal through the centre of the cube, from white to black, we also have points which contain equal, but lesser, amounts of all three primaries and these are shades of grey.

The picture here shows a diagonal slice across the cube, showing the shades of grey from black to white along the diagonal of the slice.

Some readers will be saying "I don't get white when I mix red, green and blue paints. What is going on?" This is why we said our primary colours are additive. The description has been about light emitters such as television screens and computer monitors, and the processes which occur in taking a photograph. Painting and printing use colour subtraction, which is indeed different.

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Red-cyan with grey diagonal.