Happy or sad, angry or even green with envy? We express our emotions by contracting muscles in our faces to create a smile or a frown so that others know how we feel.

Research out this week shows that our faces don't just physically change with our emotions but they also change in colour depending on how we are feeling, thanks to a large network of blood vessels under the surface of our face.

Although it is subtle, these skin tone changes around our cheeks, chin, eyebrows and nose can be subconsciously picked up thanks to cells in our eyes specialised for colour perception. This means that even if you are trying to keep a straight face, it's hard to hide your true emotions from others.

The research published in the Proceedings of the National Academy of Sciences took hundreds of pictures of people expressing different facial expressions. They then enhanced their colour digitally using a red/green or blue/yellow colour filter and separated the images by colour pattern.

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The scientists found that different emotions formed such unique patterns that they could sort the images by colour alone. Feelings of happiness showed up through an increase in red around the cheeks and temples while a blue colour surrounded the chin.

The same red cheeks with the addition of a red forehead and slightly less blue chin showed a person was surprised. Disgust, however, resulted in a very different colour set with a blue/yellow hue around the lips and a red/green hue around the forehead.

The researchers believe that the changes of colour around the face are triggered by the blood flow channelled from the nervous system depending on our state of mind.

To determine whether people can detect human emotion by colour alone, the researchers superimposed the emotional colour hues onto faces that were physically as neutral as possible with no smiles or frowns.

After showing these artificially coloured neutral faces to volunteers, they asked them to guess from a list of 18 different emotions how the person in the picture was feeling.

They found that regardless of ethnicity, skin tone or gender, even complex emotions such as sadly angry, or happily surprised were able to be correctly determined in a neutral face by colour hue alone up to 75 per cent of the time.

Next, the researchers mixed up the colours of one emotion with the physical facial expression of another, for example by overlaying a happy red-cheeked colour on top of an angry face. Although the volunteers couldn't determine what was wrong with the images, they did report that they felt something was "off" with them.

Using this information, the researchers went on to develop computer algorithms capable of determining human emotion with up to 90 per cent accuracy using facial colour alone.
Emotional recognition is already used by computers and works by using a webcam to detect key landmarks on the face such as the corners of your mouth and your eyebrows.

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Using machine learning algorithms the computer can then analyse pixels in these regions to determine facial movement and correlate it to known mapped emotions. The addition of this new colour mapping technique has the potential to dramatically increase the accuracy of how computers determine our emotion even when we try to physically hide it through keeping a straight face.

Not only does this new research open up possibilities around emotional intelligence and emotional detection for computers, but also potentially helps the cosmetics industry to create makeup that not only makes people appear younger but also happier.