It’s as basic as water faucet handles: red means hot and blue means cold. That simple fact just got more complicated, according to a surprising study in the July 3 issue of Scientific Reports  which shows that blue objects feel warmer to the touch than red ones of the same temperature. (Scientific American and Scientific Reports are part of Nature Publishing Group.)
Participants in the study were led into a pitch-dark room with a temperature-controlled plate lit up in either blue or red. Placing their hands on the surface, they were asked to state whether it felt warm. Red-colored surfaces needed to be about 0.5 degrees C hotter than blue ones before they felt at all warm to the touch.
“I was very surprised,” says Hsin-Ni Ho, a communications researcher at the Nippon Telegraph and Telephone Corporation in Japan and the study’s lead author. “I think as most people, our expectation is that red objects should feel warm and blue objects should feel cold. We get a totally reversed result. At first I was like, ‘Oh, is something wrong?’”
The result seemed to fly in the face of both our intuition about how red and blue should feel and other research into color and temperature. Earlier studies, for instance, had shown that red or blue room lighting can make a person feel warm or cold.
The reason her team’s results seem contrary, says Ho, is that unlike most previous research the new study tested how we perceive the temperature of objects we directly touch.
When it comes to touch, what we feel might be strongly influenced by our expectations. “When you look at a red object you expect it to be warm. You have something already in your mind,” Ho says. “The contrast between the expectation and actual temperature perception will influence what you feel.” Since our minds anticipate a warm red object, it takes a higher temperature for us to believe that the object is unusually hot.
To confirm their hunch about expectation, Ho’s team conducted a second experiment. Instead of coloring the heated surface, they projected red or blue light onto participant’s hands. This time, red hands made surfaces feel warm at lower temperatures than blue hands. The reason, says Ho, is that our brain expects a red hand to already be warm, so when we touch a slightly hot object we interpret it as being warmer than it is.