How do colours mix?

White light is a mixture

Sunlight looks plain and white, but it is secretly a mixture of every colour in the rainbow. About 360 years ago, a young scientist named Isaac Newton showed it. In 1665 and 1666 he darkened his room, made a small hole in the window shutter, and let a thin beam of sunlight pass through a glass prism. On the far wall appeared a long band of colours. We call it a spectrum.

Then Newton did something clever. He used a second prism to gather the colours up again, and they turned back into white light. So white light is not plain at all. It is every colour mixed together.

People before Newton usually counted five colours in a rainbow. Newton counted seven: red, orange, yellow, green, blue, indigo and violet. He also bent the band of colours into a circle in his book Opticks (1704). He is widely thought to be the first person to do that, and circles like his grew into the colour wheels that artists use.

Mixing light: red, green and blue

Here is a surprise. Your eyes do not have a separate detector for every colour. You have three kinds of colour-sensing cells, called cones, and they react best to red, green and blue light. Your brain works out every other colour from how strongly the three kinds are triggered.

That means you can trick your eyes. Shine a red light and a green light onto a white wall, and where they overlap you see yellow. Red and blue make magenta, a bright pinkish purple. Green and blue make cyan, a bright turquoise. Shine all three together and you get white. Mixing light makes it brighter, so this is called additive mixing.

Phone and computer screens use this trick. Look very closely at a screen and you will find tiny dots of red, green and blue light. From far enough away, your eyes blend the dots into every colour you can see.

Mixing paint: the opposite

Paint works the other way around. Paint does not make light. It reflects light. The coloured powder inside paint is called pigment, and it soaks up (absorbs) some of the colours in the light that hits it and bounces the rest back to your eyes. Yellow paint, for example, soaks up blue light and bounces back red and green light, and your eyes see that mix as yellow.

Now mix two paints in a pot. Each paint soaks up its own colours, so the mixture soaks up more light, and less is left to bounce back. That is why mixing paints makes colours darker, and why mixing lots of colours together makes a dull, muddy brown. Because paint takes colours away from the light, this is called subtractive mixing.

The colour wheel, and a secret about primaries

At school you probably learnt that red, yellow and blue are the primary colours, the ones you cannot make by mixing. Mixing two of them gives a secondary colour: red and yellow make orange, yellow and blue make green, and blue and red make purple. Painters used red-yellow-blue colour wheels for most of the 1800s, and they are still a handy way to learn how colours are related.

Here is the honest bit. Scientists have long pointed out that red, yellow and blue are not the true primaries. For mixing pigments, inks and filters, the starting colours are cyan (a bright turquoise), magenta (a bright pinkish purple) and yellow. Mix cyan and yellow and you get a bright green. Mix magenta and cyan and you get a rich blue. Mix yellow and magenta and you get a strong red.

That is why printers use cyan, magenta and yellow ink, plus a fourth ink: black. In theory the three colours should mix to black, but real inks are not perfectly pure, and they make a muddy brown instead. So printers add black ink. It is called "K", short for "key colour", and the whole set is known as CMYK.

Colour from the Australian earth

Aboriginal artists have long made paint from coloured earth. In traditional bark painting, the palette is mostly four colours, red, yellow, white and black, made largely from minerals called ochres. Artists mix the ochre pigment with a sticky binder so that it stays put on the surface. Scientists who tested old bark paintings found juice from orchids used as a binder in two paintings from the late 1800s.

Ochre is a pigment, so it works like the paint in your art room: it soaks up some light and bounces the rest back to your eyes. The National Museum of Australia looks after pieces of red ochre that seven women from Papunya, who are Anmatyerre, Luritja, Pintupi and Warlpiri people, used in 1990 to paint a Dreaming story at Tandanya in Adelaide.

📚 Words to know

Pigment

The coloured powder in paint or ink. It soaks up some colours of light and bounces back the rest.

Spectrum

The band of colours you get when white light is split apart, like a rainbow.

Primary colours

A small set of starting colours that can be mixed to make many others.

Absorb

To soak up. A blue paint absorbs some of the colours in light and reflects the rest.

Ochre

A coloured earth, usually red or yellow, that artists turn into paint.

🧪 Try this: mix paint, then spin light

1. Squeeze a small blob each of red, yellow and blue washable paint onto a plate, and keep a paper towel handy. Mix red and yellow, yellow and blue, and blue and red. Which colours do you get? Are they bright or dull?
2. Now mix all three paints together in a new spot. You will probably get a muddy brown or grey. The paints are soaking up more and more of the light.
3. If you have turquoise (cyan) and pink (magenta) paint or markers, try mixing them with yellow. Are your greens and purples brighter this time?
4. Now try mixing colours in your eyes. Cut a circle about 10 centimetres wide from a paper plate or card, draw six equal slices, and colour them red, orange, yellow, green, blue and purple. Ask a grown-up to push a short, sharp pencil through the middle so the disc works like a spinning top.
5. Spin it as fast as you can on a table. The colours blur together into a pale, greyish white. It will not be pure white, because paint and markers are not pure light, but it shows that colours can mix in your eyes as well as on a plate.