Ask three people to name the primary colours and you may get three answers. A painter says red, yellow and blue. A screen engineer says red, green and blue. A printer says cyan, magenta and yellow, which is also what PaneSlide uses. They are all reasonable answers to slightly different questions. This guide explains why, with a little history.
What "primary" really means
A set of primary colours is a small set that can be mixed to make a wide range of other colours. There is no single set that makes every colour people can see. Which three work best depends on how you mix:
- Adding light (screens, stage lights): each primary adds its light to the others. More light makes it brighter, and all three together make white.
- Taking light away (inks, filters, coloured glass): each primary absorbs part of white light. More layers make it darker, and all three together make black, or close to it.
The colour mixing guide explains these two kinds of mixing in detail. Here we look at how the three common sets came about.
Newton and the spectrum
In his book Opticks (1704), Isaac Newton described how a prism splits white light into a band of colours, and how those colours can be recombined into white. He arranged the colours around a circle, an early version of the colour wheel. Newton's work showed that white light is a mixture, not the absence of colour, which is the starting point for every later theory of mixing.
Red, yellow and blue: the painter's primaries
For centuries, painters and dyers worked with red, yellow and blue as their basic colours, and colour wheels built on those three became standard in art teaching. In 1810 the German writer Johann Wolfgang von Goethe published his Theory of Colours, which focused on how colours look and feel to people rather than on physics. It was influential among artists.
Red, yellow and blue (often called RYB) work reasonably well with real paint pigments, and they are easy to name and teach. Their weakness is range: mixing a bright green, a clean purple or a vivid pink from them is hard, because the pigments called "red" and "blue" each absorb a lot of light.
Red, green and blue: the light primaries
Around 1802 the English scientist Thomas Young suggested that the eye has just three kinds of colour receptor. Hermann von Helmholtz developed the idea later in the 19th century, and it is now known as the Young–Helmholtz theory. We now know the eye has three kinds of cone cell, most sensitive to long, medium and short wavelengths. The colour vision guide covers this.
If the eye has three receptors, three lights should be able to fool it into seeing many colours. In 1861 James Clerk Maxwell showed a colour photograph of a tartan ribbon, made by photographing it three times through red, green and blue filters and projecting the three images together. That is the same idea a phone or laptop screen uses today: tiny red, green and blue lights, mixed by your eye.
Cyan, magenta and yellow: the printer's primaries
Printing works by taking light away, so its best primaries are the opposites of the light primaries:
- Cyan absorbs red light, so it lets green and blue through.
- Magenta absorbs green light, so it lets red and blue through.
- Yellow absorbs blue light, so it lets red and green through.
Overlap two and only one light primary survives: cyan + magenta leaves blue, cyan + yellow leaves green, magenta + yellow leaves red. All three absorb nearly everything, leaving black.
Colour printing has a long history. In the early 1700s the painter and engraver Jacob Christoph Le Blon developed a way to print colour pictures from three engraved plates, inked in yellow, red and blue, sometimes with a fourth plate for black. His method was a forerunner of modern four-colour printing. Today's printers use cyan, magenta, yellow and black, known as CMYK. The K stands for "key", the plate that carries the fine detail, which is usually black. Black ink is added because three coloured inks together make a muddy dark brown rather than a crisp black, and because it is cheaper.
Why PaneSlide uses cyan, magenta and yellow
PaneSlide's glass is a filter, like a printer's ink. When light passes through two panes, each takes away its own part of the light. So the honest choice for glass is cyan, magenta and yellow:
| Panes | What you see |
|---|---|
| Cyan + magenta | Blue |
| Cyan + yellow | Green |
| Magenta + yellow | Red |
| All three | Black |
This gives six clear, bright colours plus black from only three panes, and every mix follows real physics. When you read a target picture, you are doing exactly what a print technician does: working out which inks, or here which panes, are layered on each spot.
So which is right?
All three sets are right for their job:
- RYB is a practical, traditional set for mixing paint and teaching art.
- RGB is best for anything that makes light: screens, projectors, stage lighting.
- CMY(K) is best for anything that filters light: printing, photographic film, and coloured glass.
Next time someone says "blue and yellow make green", you can agree with them, and add that cyan and yellow make an even brighter one.
Sources
- Wikipedia, "Theory of Colours" (Goethe, 1810; Newton's Opticks, 1704), checked 29 October 2026.
- Wikipedia, "Jacob Christoph Le Blon" (three- and four-plate colour printing, first prints 1710), checked 29 October 2026.
- Wikipedia, "Young–Helmholtz theory" and "James Clerk Maxwell" (1861 tartan ribbon photograph), checked 29 October 2026.