What Is the Most Difficult Colour to Mix, and Why?

The most difficult colours to mix are the bright, saturated ones that fall between your primaries: a vivid violet, a clean orange, a luminous turquoise, and above all a true cyan or magenta if you started with red, yellow and blue. The reason is a single rule: mixing pigments always makes the result duller than the brightest thing you put in. Any colour more saturated than your primaries is therefore unreachable, however you combine them. Which colour is hardest depends on which three you started with, and that is the real answer to the question.

• Mixing pigments always dulls, so any colour brighter than your primaries is unreachable.
• From red, yellow and blue the most difficult colours to mix are vivid violet, clean orange, and any true cyan or magenta.
• Cyan and magenta cannot be mixed from a paintbox because they are primaries themselves.
• Start from cyan, magenta and yellow and the difficult colours fall inside the reachable triangle.

Here is the rule, the culprits, and the fix.

Because pigments work by subtraction. Each one absorbs part of the light falling on it and reflects the rest. Mix two, and the result reflects only the light that neither absorbs, which is always less than either alone. Less reflected light means a darker, greyer colour. This is why a painter's palette drifts towards mud by the end of a session and why no amount of stirring will get a brighter colour out of two duller ones. With light it is the opposite: adding red and green light gives a brighter yellow. With paint, every mix is a loss.

Violet, and it is not close. Red and blue paint should give purple, and they do, but the purple is dull and brownish rather than the electric violet of a real flower. The reason is that most red paints contain a little yellow and most blue paints contain a little green, and yellow and green are the enemies of violet. Mix them and you are adding exactly the wavelengths violet needs to be free of. The same problem hits orange from a bluish red and a greenish yellow, and bright green from a warm yellow and a purplish blue.

Behind all of these sits the deeper problem: red, yellow and blue were never the true subtractive primaries. They are an old painter's approximation, and the colours they struggle with are exactly the ones they cannot reach in principle. The three colours that cannot be mixed explains the history.

Because they are primaries themselves. Cyan, magenta and yellow are the three pigments that each remove exactly one of the light primaries: cyan removes red, magenta removes green, yellow removes blue. Anything you can make by mixing is, by definition, not a primary, so a bright cyan or magenta cannot be reached from red, yellow and blue no matter how hard you try. The paintbox's blue is roughly cyan plus magenta already mixed; its red is roughly magenta plus yellow. You cannot un-mix them. This is the single most common frustration in school art rooms and it has nothing to do with skill.

Three things. Saturation: the purer and brighter the target, the fewer routes there are to it, and none of them run through a mixture. Position: colours that sit between two of your primaries need those two primaries to be clean, and most pigments are not. And gamut: every set of primaries encloses a triangle of reachable colours, and anything outside the triangle is simply impossible for that set. A printer's cyan, magenta and yellow enclose a bigger triangle than red, yellow and blue, which is exactly why printers use them.

Yes. On a screen, mixing is additive and always gets brighter, so the hard colours are the deep, saturated ones at the edges: a true spectral violet, a pure cyan, a laser green. A screen made of red, green and blue dots cannot show a colour more saturated than its own dots, so the most vivid colours of the real world, a flamingo's pink under sun, a morpho butterfly's blue, get clipped. The additive versus subtractive explainer sets out the two systems side by side.

Start from better primaries. If you want bright violets, mix from a magenta-leaning red and a cyan-leaning blue rather than a warm red and a greenish blue. If you want clean oranges, use a yellow with no green in it and a red with no blue. Better still, use cyan, magenta and yellow themselves: they enclose the widest range three pigments can, and violet, orange and green all fall inside the triangle. Or add a fourth or fifth pigment, which is what most serious painters quietly do, because three tubes were never enough.

Like three cubes. The C Cube, M Cube and Y Cube are solid acrylic filters in each subtractive primary. Overlap cyan and magenta at a window and a clean, bright blue appears that no red-yellow-blue paintbox can produce. Overlap magenta and yellow and you get the vivid red. Overlap all three and the light goes nearly dark, which is the subtraction rule made visible. The CMY Pack puts them together with the Original CMY Cube, and the Motus, an icosahedron with twenty faces, throws dozens of those overlaps at once.

Mixing pigments always dulls, so any colour brighter than your primaries is out of reach. From red, yellow and blue the hardest colours are vivid violet, clean orange and any true cyan or magenta, because the set cannot reach them in principle. Start from cyan, magenta and yellow and the difficult colours fall inside the triangle. Hold the three filters to a window and you can see the difference in a second.

See the three primaries that make mixing easy, and view colour through a different lens.