What they found is that the coffee-drinkers tended to experience the same cup of coffee differently depending on the color of the glassware they drank it from.
"The color of the mug really does seem to have an impact," said Charles Spence, head of the crossmodal research laboratory at Oxford University and one of the study's authors. "We found a particularly significant difference between the white mug and the clear one."
Specifically, the white mug was associated with a more "intense" (or bitter) tasting cup of coffee, and the clear glass mug was not. The blue mug, meanwhile, proved to be "kind of an intermediate."
The opposite was true for perceived sweetness—participants noted less sweetness when drinking from the white mug than they did when drinking from both the blue and clear glass mugs. Differences observed in the rest of the flavor points were statistically insignificant, because of the small scale of the experiment. But Spence plans to extend it to a larger group, and expects to find a similar pattern.
"I have been working for more than a decade studying the impact colors can have on the experience of food," he said. "It doesn't just happen in laboratories—it happens in restaurants, too."
Indeed, the idea that the color of a coffee mug can change more than merely aesthetics is actually part of a growing pool of research detailing how colors impact the experience of food.
A 2002 study, for instance, found that certain colors were consistently associated with specific tastes.
Similarly, a study from 2010 details how the color of food can be associated with crossmodal effects (when one sense affects another—in this case, vision and taste) that make it taste different, even though its taste profile is technically unchanged. Research published this year shows that the color of a plate "had a clear effect on flavor intensity, sweetness and liking scores." And the color red, specifically, has even been associated with lower levels of consumption.
The reason for the unlikely influence colors have on how people experience food is unclear. One explanation is that we never see colors by themselves; instead, we see them in reference to other colors.
A red strawberry looks more red when placed against a white plate than it does against, say, a black one. And there are associations with that redness that in turn can affect taste, Spence explains. "Red might indicate heightened sweetness, because red fruits tend to be more ripe than green fruits, he said." And that indication can be enough to trigger a tangible difference in taste.
It's also possible that certain colors are tied to certain experiences. "When we see something new, our brain might be predicting what the experience will be like based on past experiences, and thereby altering the experience," Spence said. "But we can't say for sure. Not yet."
In the case of coffee, specifically, the researchers have a hunch. The color brown, they believe, might be something people associate with bitterness. "The white mug may have influenced the perceived brownness of the coffee and this, in turn, may have influenced the perceived intensity (and sweetness) of the coffee," the researchers wrote. That would help explain why clear, glass coffee mugs, which dilute the color, tended to have the opposite effect.