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An ink that changes colour when exposed to light, like an octopus does to match its surroundings, could one day be used for automatic camouflage.

Most colour-changing inks and materials use chemical reactions, but these can be unstable and difficult to control. Instead, octopuses use special muscles to push coloured ink particles to the surface of their skin.

Now, at the University of Hong Kong and his colleagues have developed an ink that can similarly display different colours by moving dyed particles in response to light exposure. The ink is made up of particles of titanium dioxide, each with different dyes and varying light responses, arranged in a solution.

When light from a standard projector is shone on material containing the ink, a chemical gradient causes some ink particles to rise to the surface and others to fall. “Like with oil and water, [the particles] separate and float to the top, and that is because they’re coloured,” says Tang. “You can change their colours accordingly and they mimic whatever the colour you’re actually shooting on them.”

Tang and his team formulated their ink with three colours – cyan, magenta and yellow – used in the common CMY colour scheme. They then used a modified projector to display semi-permanent images, such as children’s paintings, using the ink. They found that the images stayed stable for around half an hour before the ink remixed.

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With further research, this could one day be used to provide automatic camouflage. “In the forest, everywhere is green, so your clothes or the material should receive that kind of green-colour light around and then it becomes green,” says Tang. “Navigating the desert, everywhere is yellow, then [the ink] becomes a yellowish colour.”

To be used in such a setting, however, the ink will need to maintain its desired colour for longer than half an hour before remixing, says Tang.

As the ink rearranges itself without electricity, it could be used in a wide range of applications, such as updating signs without requiring high amounts of energy, says at the University of Reading, UK. “[The ink] doesn’t have any electrodes in it, you just have an external source to change the colour and set it in place,” he says. “That adaptive octopus camouflage idea makes it far more practical than it would be otherwise.”