The Fascinating CMY Cube: How It Works and Changes Colour With Angle

The CMY Cube works by subtractive colour mixing: each face is a filter that removes one part of white light, and wherever two faces overlap you see the colour that is left. You have seen the Original CMY Cube online or in person and you are wondering how it works. Here is the science behind this colourful object, one colour at a time, and what happens when they combine.

• How it works in one line: cyan, magenta and yellow faces each absorb one primary of light, and overlapping them subtracts more.
• Cyan over yellow makes green, magenta over yellow makes red, cyan over magenta makes blue.
• All three together absorb almost everything, which is why the centre goes dark.
• It is the same principle every colour printer uses, at a scale you can hold.

Here is the longer version.

There are two ways to mix colour, and they behave in opposite ways. Additive mixing adds wavelengths of light together: red, green and blue light overlap to make white, which is how every screen you own works. Subtractive mixing starts with white light and removes wavelengths from it. Pigments, inks and coloured filters all work this way, which is why subtractive mixing is the technique behind printing, where only four inks are used: cyan, magenta, yellow and black.

The cube has six faces in three colours: two cyan, two magenta, two yellow. Look straight through one face and you see one primary. Tilt it so your line of sight passes through two faces and the light has to survive two filters in a row. Cyan removes red light, so what comes through a cyan face is green plus blue. If that light then passes a yellow face, which removes blue, only green is left. That is why cyan over yellow gives green, magenta over yellow gives red, and cyan over magenta gives blue. Turn the cube and the overlaps change, and new colours appear.

Because the combination of faces between your eye and the light source changes with the angle. Each colour of light has its own wavelength, and each face of the cube absorbs a particular band of wavelengths. As you rotate the cube, the path light takes through it crosses different faces, so different bands are removed and the colour you see shifts. Two faces give a secondary colour. Three faces in a line absorb red, green and blue together, leaving almost nothing, which is why the middle of the cube can look nearly black.

Nothing is being created; things are being removed. We see objects because light reflects off them or passes through them and reaches the eye. White light contains every visible wavelength. When it enters a cyan face, the dye absorbs the red photons and lets the rest continue. Each further face absorbs its own band. Whatever reaches your eye is white light with pieces missing, and your brain names the remainder as a colour. The CMY Cube is, in effect, a physical demonstration of how a colour space works, with your hand as the controller.

Almost everywhere colour is put onto a white surface. Inkjet and offset printing lay tiny dots of cyan, magenta and yellow side by side and on top of one another to reproduce photographs. Textile printing uses the same primaries for heat-transfer patterns. Painters mix pigments subtractively. Screens work the other way, in RGB, which is why a photo on your phone has to be converted to CMY before it can be printed on paper.

In theory, cyan, magenta and yellow together should give black. In practice inks are imperfect, so all three together give a muddy dark brown and use a lot of ink doing it. Printers add a fourth ink, black, called K for 'key', to get crisp blacks and deeper shadows. It is a practical shortcut, not a fourth primary.

The cube can only subtract colours that are already in the light. For the fullest effect, view it in plenty of pure white light: a bright room, a window, a lightbox. White contains every wavelength, so every subtraction produces a clean result. Under coloured light, warm lamps or a sunset, some wavelengths are missing before the cube gets to work, and some of the mixes will not appear. That is not a fault. It is the physics showing you what your light source is made of.

In every printed page, every rainbow and every smart bulb. LED smart bulbs mix red, green and blue light additively to make any colour of the spectrum. Rainbows split white light into its wavelengths through refraction and reflection in raindrops. And anyone who paints with watercolours is mixing subtractively every time two washes overlap, which is why the CMY Cube is such a useful tool for understanding which colours combine to make which.

The Original CMY Cube is a solid, optically clear acrylic cube that turns this whole article into something you can watch happen in your hand. For the primaries as separate pieces, the CMY Pack gives you a cyan, a magenta and a yellow cube plus the Original, so you can build every overlap yourself. The science page goes wavelength by wavelength if you want the long version.

How it works: each face of the CMY Cube filters out one primary of light, overlapping faces filter out more, and what reaches your eye is the colour that survives. Cyan and yellow make green, magenta and yellow make red, cyan and magenta make blue, and all three together go dark. It is the same subtractive mixing behind every colour printer, and it changes every time you turn the cube.

Shop the CMY Cube, and see light through a different lens.