The flappy skin on the turkey's face and throat could help save human lives in a terror attack or a chemical spill -- or maybe even from cancer.

For millennia, the plump fowl has used the wattle to show agitation or excitement by changing it from red to blue, or blue to white -- an ability that has earned it the name "seven-faced bird" in Korean and Japanese.

Intrigued by the colour trick's potential as a toxin alarm, engineers from the United States and South Korea decided to take a closer look at turkey skin, and found it contains bundles of light-reflecting collagen fibres, interspersed with a dense array of blood vessels.

As the arteries swell or contract, so do the collagen bundles -- changing the angle at which they scatter incoming light and thus the skin's perceived colour, the scientists discovered.

The team then used harmless viruses known as M13 bacteriophages, which self-replicate, to create films containing similar fibre bundles to mimic the structure of turkey skin.

The biofilms changed colour "immediately" when exposed to different chemical vapours, becoming an easy-to-see sensor of danger, even to the naked eye.

Compounds detected included hexane, isopropyl alcohol, ethanol and methanol... even TNT vapour.

The team also developed a mobile application, dubbed iColour Analyser, to distinguish between hues -- thus identifying the chemical specific to each.

"Our system is convenient, and it is cheap to make," said Seung-Wuk Lee of the University of California, Berkeley, who led the study published in the journal Nature Communications on Tuesday.

"This technology can be adapted so that smartphones can help analyse the colour fingerprint of the target chemical."

Many scientists are working on the development of sensors that give colour readings, as they are easy to use, but most are not selective enough to be tuned for specific chemicals.

The new method was reactive and sensitive to an array of different chemicals at concentrations as weak as 300 parts per billion -- and could be further refined through genetic engineering, the authors said.

"Our tunable, colourimetric sensors can be useful for the detection of a variety of harmful toxicants and pathogens to protect human health and national security," said the study.

"In the future, we could potentially use this same technology to create a breath test to detect cancer and other diseases," added Lee.