When it comes to deciding what to eat, colours affect our brains like an upside down traffic light.

Sight is not the only sense helping us to choose what to eat.

We also rely on colours to decide about the quality of a food and for estimating the amount of calories.

The human brain is naturally designed to prefer certain colours over the others, choosing red foods such as meat over green looking foods, like vegetables, new research shows.

The researchers think the colour preferences could be a result of evolutionary forces helping us to choose nutritious and edible foods.

Professor Raffaella Rumiat, a study co-author, said:

“According to some theories, our visual system evolved to easily identify particularly nutritious berries, fruits and vegetables from jungle foliage.”

Three receptors in the retina of the eye are responsible for colour vision.

One is sensitive to the colour red, the other to green and the third to the colour blue.

Consequently, the human eyes can see a larger number of colours than many animals.

Professor Rumiat explained:

“We are particularly efficient at distinguishing red from green.

This sophistication testifies to the fact that we are “visual animals,” unlike others, dogs, for example, who depend on their sense of smell.

It is mainly the color of food that guides us, and our experiments show how.”

In general we look for nutrition, high protein and high calorie content  in foods.

Dr Francesco Foroni, first author of the study, explained:

“In natural foods, color is a good predictor of calories.

The redder an unprocessed food is, the more likely it is to be nutritious, while green foods tend to be low in calories.

Our visual system is clearly adapted to this regularity.”

Dr Giulio Pergola, study co-author, said:

“The participants in our experiments judged foods whose color tended towards red as higher in calories, while the opposite was true for greens.

This is also true for processed, or cooked foods, where color loses its effectiveness as an indicator of calories.”

A number of studies show that we favour cooked foods over natural foods and this has been supported by this research.

Professor Rumiat explained:

“Cooked foods are always preferred because, compared to natural foods, there is more nutrition for the same quantity.

With cooked foods, however, the dominance of red over green no longer provides reliable information, which might lead us to believe that the brain would not apply the rule to processed foods. On the contrary, it does, which hints at the presence of ancient evolutionary mechanisms from before the introduction of cooking.”

The study was published in the journal Scientific Reports (Foroni et al., 2016).

 Red and green apples image from Shutterstock