YOUR EYES
Three kinds of cone
About 6 million cones in each retina come in three types, tuned to short, medium and long wavelengths of light.
Colour factsStart
HOW WE SEE COLOUR
Light adds.
Ink subtracts.
Every colour you see is your eyes and brain doing sums.
Your retina has three kinds of colour-sensing cell. Screens mix red, green and blue light to tickle them; printers lay down cyan, magenta and yellow ink to block parts of the light instead. Here is how it fits together, from Newton's prism to the colour wheel on your screen.
Plain English, with the sources linked.
R G BC M Y K
Try the colour mixerKeep in mind
- Three cones
- Light adds
- Ink subtracts
- The brain does the rest
THE SHORT ANSWER
Three cones, two ways to mix.
Screens add light together.
Paint and print take light away.
SCREENS · RGB
Light adds up
Red, green and blue light add together. Overlap all three and you get white.
See the sums
PRINT · CMYK
Ink takes away
Cyan, magenta and yellow inks absorb part of the light, so each one you add makes the colour darker.
Mix some coloursA QUICK COLOUR MIXER
What do two colours
make together?
Pick a pair. The first three are coloured lights, as on a screen. The last three are inks or paints on white paper.
Light and ink give different answers because light adds wavelengths while ink absorbs them. The ink results are idealised: real paints are rarely this clean.
TWO KINDS OF COLOUR SUM
Add light,
subtract ink.
The same three-colour trick,
run in opposite directions.
R + G + B = white
Screens, projectors, stage lights
White light can be treated as the sum of red, green and blue. A screen starts dark and adds these three in different strengths to stimulate your three kinds of cone.
ADDITIVE · RGB
C + M + Y ≈ dark
Printing, paint, photographic film
Cyan absorbs red, magenta absorbs green and yellow absorbs blue. Overlap all three and almost all the light is absorbed, which is why printers also carry a black ink, the K in CMYK.
SUBTRACTIVE · CMYK
The two systems are mirror images: each pair of inks leaves one of the light primaries, and each pair of lights makes one of the ink primaries.
TRY IT AT HOME
Four ways
to see it for yourself.
Step 1
Look closely at a screen
Use a magnifying glass on a white part of an older screen and you may see the tiny red, green and blue dots that add up to white.
Step 2
Overlap two paints
Brush a thin wash of yellow, let it dry, then glaze cyan over half of it. Where they overlap, you should see green.
Step 3
Make a rainbow
Stand a glass of water or a prism in a sunny window over white paper and look for the band of colours, as Newton did.
Step 4
Design with everyone in mind
About 1 in 12 men see colour differently, so never let red versus green carry a message alone. Add labels, shapes or patterns too.
COLOUR FACTS
Five things worth
seeing clearly.
A narrow slice of light
Visible light runs from about 400 nanometres at the violet end of the spectrum to about 700 nanometres at the red end, and the limits vary a little from person to person.
Rods outnumber cones
The human retina has about 120 million rods, which work in dim light, and about 6 million cones, which give us colour.
Three is the magic number
Cones come in three types, short, medium and long wavelength. Thomas Young first recognised that colour vision is three-channelled at the start of the nineteenth century.
Newton bent the rainbow into a circle
Newton used prisms to argue that white light was made of seven colours, and published his findings in Opticks in 1704. The colour circle he first devised is still widely used in colour design.
Not everyone sees the same colours
Colour vision deficiency affects about 1 in 12 men and 1 in 200 women. Most have trouble telling apart shades of red, yellow and green.
QUICK QUESTIONS
Colour, answered.
Short answers to the questions people ask most.
Why do screens use RGB but printers use CMYK?
A screen gives off light, and coloured light adds: red, green and blue together make white. Paper only reflects light, and inks work by absorbing part of it. Cyan absorbs red, magenta absorbs green and yellow absorbs blue, so printers start from white paper and take colours away.
How many colour-sensing cells are in the eye?
The human retina has about 6 million cones, which handle colour, and about 120 million rods, which work in dim light. Cones come in three types, sensitive to short, medium and long wavelengths.
Who invented the colour wheel?
Isaac Newton is credited with the first colour circle. He split white light with prisms, argued it was made of seven colours, and set out his findings in Opticks, published in 1704. The colour circle is still widely used in colour design.
How common is colour blindness?
Colour vision deficiency affects about 1 in 12 men and 1 in 200 women, according to NI Direct. The most common type makes it hard to tell apart shades of red, yellow and green. Blue-yellow deficiency is rare.
Can colour blindness be tested or cured?
An optician can test colour vision, for example with the Ishihara test of coloured dots; it is not usually part of a routine eye test, but you can ask. There is no cure for the inherited form, though most people adapt to it over time.
Destination
Where the facts come from
- Cones and colour vision (Purves et al., 2001)NCBI Bookshelf · Neuroscience, 2nd ed.
- Neuroscience for Kids: the retinaUniversity of Washington
- Colour: the visible spectrumEncyclopaedia Britannica
- Subtractive colour mixing (Ingersoll Physics Museum)University of Wisconsin–Madison
- Isaac Newton, Opticks (1704)Whipple Library, University of Cambridge
- Colour vision deficiency (colour blindness)NI Direct (UK government)
Updated