Colour Constancy

A colour illusion where three sets of grey eyes all appear to be coloured differently.

How to Use It

Look closely at the eyes—do they appear to be the same or different? Ask a staff member to demonstrate that they are the same with a grey card.

What Science Tells Us

It is highly beneficial for our perception of colour to remain fairly stable between different lighting conditions. For example, objects change colour (get darker) in shadow, but we don’t tend to recognise this. Cues from the light —highlights, shadows, general colour cast—all influence our perception of the world around us. A surface under blue lighting will be assumed to be more yellow (‘less blue’) than it appears, and our brains automatically adjust to account for this.

The images of the eyes here have coloured overlays, changing how ‘black’ the black areas are, and the colour of the highlights in the eyes. This leads our brains to assume that the lighting is similarly coloured. To correct for this, our brains ‘subtract’ the colour of the light from the colour of the surface. This results in neutral greys appearing to be the complementary colour of the light source.

Things to Try or Ask

Use a piece of grey card or fabric to check the colour of the eyes.

Take a photograph of the eyes and zoom in – what colour are they?

Finding the Science in Your World

At sunset the light from the sun is very orange/red, and everything around you changes colour. Most of the time we don’t notice, due to colour constancy. For this reason photographers also often adjust the light source (or colour temperature) setting on their camera so that colours are rendered properly.