Think you can trust your eyes? This simple trick makes you see colours that don’t exist

Can you trust what your eyes are showing you? Most of the time, the answer seems obvious. But a simple image of a frog and a pig has been making people question that assumption. Stare at it for several seconds and something strange happens: colours appear in places where there are no colours at all. The viral illusion shared by British magician and comedian Pete Firman looks like another clever magic trick. This time, however, the explanation has less to do with sleight of hand and more to do with the tiny light-sensitive cells inside our eyes. Recent research from the University of Southampton has offered a clearer explanation of exactly why the effect occurs.

A Pink Frog, Green Pig and One Black Cross

Firman's demonstration begins with two cartoon animals deliberately shown in unusual colours. The frog appears purplish-pink, while the pig is bright green. A black cross sits between them. Viewers are asked to keep their eyes fixed on the cross, avoid blinking if possible and maintain roughly the same viewing distance. After several seconds, the colours disappear and the animals are shown in black and white. Except that they may not look black and white. For a brief moment, many viewers see something closer to the animals' familiar colours: the pig appears pink and the frog green. Firman tells viewers he has made them see colours that aren't actually present in the final image. And in a sense, he has; although there is a scientific explanation.

Why Do You See Colours That Aren’t There?

The phenomenon is known as a colour afterimage. It occurs when someone looks at a colour for an extended period and then shifts to a neutral or differently presented image. An illusory colour can briefly remain even though that colour is no longer physically displayed. Scientists have debated the precise mechanism behind colour afterimages for years. Some explanations focused on the eye's photoreceptors, while others attributed the effect to later processing within neural pathways or the brain.


Research by Dr Christoph Witzel, Associate Professor of Psychology at the University of Southampton, has provided stronger evidence for where the illusion begins. And the answer appears to be right inside the eye.

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Your Cone Cells Are Behind the Trick

Cone photoreceptors are cells in the retina that allow us to perceive colour. When exposed continuously to particular colours, their responses adapt. Witzel conducted a series of experiments measuring the exact colours people experienced as afterimages and compared the results with computational predictions for different stages of the visual system.

Across the experiments, the results were consistent with adaptation occurring in the cone photoreceptors rather than an opposing-colour mechanism elsewhere in the visual system. The research was published in Communications Psychology in 2025. In other words, when Firman's colourful cartoon suddenly turns greyscale, your eyes have already adapted to what they were staring at. The resulting afterimage can temporarily place colours into a picture that contains none.

It Isn't Really About Frogs and Pigs

There is another interesting twist. The illusion does not depend on your brain knowing that frogs are generally green or pigs are commonly depicted as pink. The same basic effect can be produced using simple coloured shapes. That distinction matters because it means viewers are not simply remembering what colour an animal "should" be and mentally filling it in. The illusion is rooted in the way the visual system responds after prolonged exposure to colour. Witzel's findings are especially interesting because colour afterimages are hardly a new discovery. What his research adds is experimental evidence supporting cone-cell adaptation as their origin.

A Simple Trick With a Bigger Lesson

Firman's viral demonstration lasts only seconds, but it highlights something we rarely think about: seeing is not as straightforward as opening our eyes and recording the world exactly as it exists. Our visual system is constantly responding and adapting to the information reaching it. Usually, that process helps us navigate the world without noticing the complicated biology working behind the scenes. Occasionally, an optical illusion exposes the machinery. The pig never actually turns pink and the frog never becomes green. Yet, for a few fleeting seconds, your own eyes can make you swear that they did.