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Curiosity · Light and colour

Mixing light and colour

When red and green light overlap, why do you get yellow and not brown? Find out how screens make millions of colours from just three, and how your eye "calculates" colour.

  • 15 min
  • 2 simulations
  • Mini quiz

Updated:

Preview

The question

There is a picture of a bright yellow sun on your phone screen. If you used a drop of water as a magnifying glass and looked closely at that yellow, what would you see? Surprise: there is not a single yellow dot there. There are only tiny red and green lights lined up side by side. So how does your eye get yellow out of them?

Try it yourself

No account needed: change the values and check your answer. Some simulations ask for a prediction before they start; it doesn't matter whether it's right or wrong.

Colour and light

Make yellow with light

Tap each light's button to turn it on or off. Look at the middle region on the wall where the circles overlap; when you find the colour you want, press Check.

Prediction

Predict first

If you overlap red and green light on a white wall, what colour appears in the middle?

It doesn’t matter whether you’re right; the simulation opens once you choose.

Colour and light

Play with ratios: orange light

Use the sliders to choose the brightness of each spotlight. The colour in the middle region and the colour name below it will guide you.

Prediction

Predict first

Can you make orange using only red, green and blue light?

It doesn’t matter whether you’re right; the simulation opens once you choose.

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Embed the simulation on your site

Paste this code into the HTML of your blog or school site. Visitors try the simulation without an account; no progress is saved.

The embedded page sets no cookies and does no tracking. A small link back to Kuazara sits underneath.

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Frequently asked

5 questions

Why do red and green light make yellow and not brown?

Lights add to each other. The mix stimulates the red-sensitive and green-sensitive cones in your eye together, just as real yellow light does; your brain cannot tell the difference and says "yellow". Brown is a habit that comes from mixing paints.

Why does mixing paint give a different result from mixing light?

Paint makes no light; it absorbs part of the light that falls on it. The more paint you add, the more colours are absorbed and the darker the mix gets. This is subtractive mixing. Lights add to each other, so where they overlap it gets brighter: additive mixing.

Is there really yellow light where a screen looks yellow?

No. In an area that looks yellow, only the red and green subpixels are lit and the blue one is off. The light reaching your eye is different from real yellow light, but it looks the same to you. Colours that are different as light but look the same to the eye are called metamers.

Isn't white light colourless?

No. White light is a balanced mix of all the colours of the rainbow; a prism splits it into its colours from red to violet. On a screen, white appears when the red, green and blue subpixels all shine together at full brightness.

How can a screen show millions of colours with only three?

The brightness of each subpixel is set separately. If each channel has 256 brightness levels, that gives 256 × 256 × 256 = 16,777,216 different mixes. For example, with red full, green at half and blue off, you see orange.