What is colour, really?

Colour begins with light. White light, the kind that pours through your window, is a mixture of every colour at once. In 1666, Isaac Newton passed sunlight through a glass prism and watched it fan out into the visible spectrum: red, orange, yellow, green, blue, indigo, and violet. He proved that colour isn't added to light. It's already in there, waiting to be separated.

Each colour corresponds to a different wavelength of light. Red waves are the longest, violet the shortest, and everything you have ever seen sits somewhere in between. When light strikes an object, some wavelengths are absorbed and others bounce back to your eye. The wavelengths that escape are the colour you see. A lemon is yellow because it swallows blue light and lets the rest go.

Bending light: refraction and reflection

Light travels in straight lines until it changes medium. When a beam passes from air into glass, water, or acrylic, it slows down and bends. That bending is called refraction. When light bounces off a surface instead of passing through, that's reflection. Almost everything light does in the world is some combination of the two.

Refraction hides a secret: different wavelengths bend by different amounts. Violet light, with its short wavelength, bends the most. Red, with the longest, bends the least. So when white light enters a prism or a crystal, its colours take slightly different paths and fan apart. By the time the beam exits, the colours have separated into a spectrum. That separation is called dispersion, and it's exactly what Newton saw on his wall in 1666.

So where do rainbows come from?

A rainbow is the same experiment performed by the sky. Every raindrop is a tiny sphere of water, and when sunlight strikes one, three things happen in quick succession: the light refracts as it enters the drop and the colours begin to separate, it reflects off the back of the drop, and it refracts again on the way out, spreading the colours further apart.

Each drop sends only one colour to your eye; the full arc is the work of millions of drops, each delivering its own slice of the spectrum. The geometry is strict. The light leaving a drop makes an angle of about 42 degrees with the direction the sunlight was travelling, so the only drops that can send light to your eye are the ones sitting 42 degrees away from the point directly opposite the sun (find it fast: it's where the shadow of your head is). All the directions that sit 42 degrees from that point trace a circle in the sky, and that circle is the rainbow. You rarely see the bottom half because the ground gets in the way. And a double rainbow? That's light reflecting twice inside each drop before leaving, which sends the second, fainter arc out at about 51 degrees with the colour order flipped.

How does a CMY Cube work? Subtractive colour mixing.

Cyan, magenta, and yellow are the three subtractive primary colours. Each one is defined by what it takes away: cyan absorbs red light, magenta absorbs green, and yellow absorbs blue. Whatever survives the journey through the filter is the colour that reaches your eye.

A CMY Cube puts all three filters on one object. Look through a single face and you see that face's colour. Look through two faces at once and the filters stack, each subtracting its share of the spectrum:

This is subtractive colour mixing, the same physics behind paint, ink, stained glass, and colour printing. Turn the cube and the combinations shift continuously, which is why it never looks the same twice. From three colours, dozens emerge.

Subtractive vs additive: why screens are different

There are two ways to make colour, and they are mirror images of each other. Your phone screen uses additive mixing: it starts with darkness and adds red, green, and blue light. Add all three and you get white. Pigments, inks and CMY Cubes use subtractive mixing: they start with white light and take wavelengths away. Subtract everything and you approach black.

It's the answer to a question every child asks and most adults can't answer: why does mixing paints give you different colours than mixing light? Now you can demonstrate it from your desk.

What does the K stand for?

In theory, cyan plus magenta plus yellow makes black. In practice, mixed inks produce a muddy dark brown, so printers add a fourth ink: K, for "key". That's CMYK, the model behind almost everything ever printed. The K Cube explores that shadowy end of the spectrum, a deep translucent black that studies the absence of colour with the same precision as its CMY siblings.

The five Platonic solids, in translucent colour

Over 2,000 years ago, Plato described five perfect 3D shapes: the only solids whose faces are identical regular polygons meeting identically at every corner. He was so taken with them that he matched each to a classical element. There are exactly five, there will never be a sixth, and our geometric range brings each one to life.

From flat to solid: 2D vs 3D

Every Platonic solid has a flat cousin. A regular polygon is the 2D version of the same idea: every side the same length, every angle the same size. An equilateral triangle, a perfect square, a regular pentagon. And here's the thing about 2D: the list never ends. You can draw a regular hexagon, a regular octagon, a regular 17-gon, a regular 1,000-gon. Flat space is endlessly generous.

Step up one dimension and that generosity vanishes. Build with those same perfect polygons in 3D, demanding the same regularity at every corner, and the infinite list collapses to exactly five solids. That cliff edge between dimensions is one of the most surprising results in geometry, and the proof is simple enough for the next section.

In 2D: regular polygons go on forever

Triangle, square, pentagon, hexagon, 17-gon, 1,000-gon...

In 3D: a flat net folds into a solid

Six squares become a cube. Only five solids are possible.

The bridge between the two worlds is the net: unfold a cube along its edges and it flattens into a cross of six squares, the 3D solid laid out in 2D. Print one, fold it, and you've built a Platonic solid with your hands, which is a classroom favourite for a reason. And if you're wondering whether the pattern continues upward: in four dimensions there are exactly six regular shapes, and from five dimensions onward, only three. Three dimensions, with its five perfect solids, is the genuinely interesting place to live. Lucky us.

What makes a solid Platonic? And why only five?

The membership rules are strict. A Platonic solid must be convex (no dents), every face must be the same regular polygon (all sides equal, all angles equal, like an equilateral triangle or a perfect square), and the same number of faces must meet at every corner. Miss any one of those and you're out of the club.

Why only five? It comes down to what can happen at a corner. To fold flat shapes into a 3D corner, the angles meeting there must add up to less than 360 degrees. Reach 360 exactly and the shapes lie flat as a tile. Go over and they can't close at all. Run through the possibilities and the list exhausts itself:

• Equilateral triangles (60° each): three at a corner makes 180° (the tetrahedron), four makes 240° (the octahedron), five makes 300° (the icosahedron). Six makes 360°, which lies flat. Dead end.
• Squares (90° each): three at a corner makes 270° (the cube). Four makes 360°. Flat. Dead end.
• Regular pentagons (108° each): three at a corner makes 324° (the dodecahedron). Four is over 360°. Impossible.
• Regular hexagons (120° each): three already makes 360°. They tile your bathroom floor beautifully, but they will never fold into a solid.

3 hexagons = 360°

Lies flat forever

3 pentagons = 324°

The gap closes into 3D: a dodecahedron begins

Count them up: three solids from triangles, one from squares, one from pentagons. Five. Exactly five, forever. No new discovery, no clever engineering, no future mathematics will ever add a sixth. Euclid proved it as the grand finale of his Elements around 300 BCE, and it remains one of the tidiest results in all of geometry.

Plenty of other beautiful solids exist, they just bend the rules. Allow more than one kind of regular face and you get the Archimedean solids, like the truncated icosahedron, which mixes hexagons and pentagons and is better known as a football (or a soccer ball, depending on where you kick it). Allow non-identical corners and you get prisms and antiprisms. The Platonic five are the only shapes that follow every rule at once, which is why they've fascinated thinkers for over two millennia, and why we made all five translucent: perfect geometry deserves to be seen through, not just looked at. Meet all five.

How a CMY Cube is made

Each cube begins as optically clear acrylic, diamond polished to perfection so light enters and exits without distortion. Every face is then sealed with colour, and that seal is what performs the mixing. The exact manufacturing process is proprietary, which is a polite way of saying it stays our secret.

The colour seal is also why we're particular about care: no water, no sprays, no chemical cleaners. A dry microfibre cloth (or our CMY Klean) keeps the physics brilliant.

Why aren't we called "CMY Kubes"?

We get asked this a lot, and the answer is hiding in Part One. The K in CMYK is the printer's confession: mixed inks can't reach a true black, so the print industry adds a fourth ink to finish the job. The cube never needs that help. Hold one to the light, look through all three faces at once, and cyan, magenta, and yellow filter the spectrum all the way down to black on their own. There's no K in the cube, so there's no K in the name.

CMY isn't a typo or a branding quirk. It's the whole point: three colours, pure subtractive mixing, nothing added. The closest we come to a K is The K Cube, our deep translucent black, and even that exists to explore what happens when colour leaves the room. Also, between us, spelling cube with a K would have haunted us forever.

Don't take our word for it

Vsauce, one of the world's most-loved science channels, featured the CMY Cube in a dedicated Short exploring subtractive colour mixing, with a special thanks to "the team at cmycubes.com". The cube has also appeared alongside the Curiosity Box, Vsauce's science subscription box. Science communicators reach for it for the same reason teachers and parents do: it turns an abstract idea into something you can hold, turn, and test for yourself.

Beyond the screen

Here's a secret the display industry doesn't advertise: no screen can show you every colour your eye can see. Colour scientists map human vision as a horseshoe-shaped region called the chromaticity diagram, first standardised in 1931. Every screen mixes its colours from three points of red, green, and blue light, so the colours it can reproduce are trapped inside a triangle drawn between those three points. The region is called the screen's gamut, and everything outside it, including the deepest spectral cyans and greens, simply cannot appear on a display.

A CMY Cube sits on the other side of that limitation. It doesn't render colour from three points of light. It filters real, continuous sunlight, so the colours that reach your eye are bound by physics, never by a gamut. The difference between seeing a spectral cyan on a monitor and through a cube held to the morning sun is the difference between a photograph of a place and standing in it.