Remember the grade-school game ‘Red Light, Green Light?’ You had to run across the playground until someone said ‘red light,’ and then you had to freeze, unable to move again until you heard ‘green light.’

Nice idea, thought some researchers at Massachusetts General Hospital (MGH), could the same go-stop training manipulate the way people eat?

It turns out it can. Dr. Anne Thorndike, a physician in general medicine at MGH, decided to run an experiment in the hospital cafeteria on her fellow colleagues. (Doctors, by the way, tend to have some pretty appalling eating habits, and hospital cafeterias are often to blame.) Thorndike and her team color-coded all of the food items for sale: the healthiest foods (fruits and vegetables and lean meats) got green labels; the moderately healthy ones, a yellow label; and the least healthy options (chips, sugary sodas and fried foods) were branded with a red label.

The cafeteria registers were programmed to record how many items in each category were purchased. It turned out that the color codes made a difference. Sales of red-tagged items decreased 9.2%, with red beverage purchases dropping by 16.5%. Sales of green-tagged items increased by 4.5%, with a 9.6 increase in green beverages.

Thorndike and her team then conducted another experiment, based on the principles of consumer buying behavior. They rearranged the placement of certain foods — beverages, pre-made sandwiches and chips (the kind of stuff harried consumers tend to grab and go) — so that the healthier stuff was at eye level, and the less healthy stuff above or below.

The researchers placed water, low-fat dairy and diet drinks, at eye level in the refrigerated cases, and moved red- and yellow-tagged sodas to the bottom shelves. They also featured green-labeled sandwiches at eye level, and shifted those with fattier meats and cheeses either above or below them.

The product placement resulted in an additional shift in sales consistent with the initial pattern. After the change in location, sales of red items dropped another 4.9%. Sales of green items actually decreased as well, by 0.8%, but sales of green beverages increased by another 4%.

The beauty of the program is its simplicity — it doesn’t take much for a busy doctor, nurse or any other consumer to comprehend the difference between a red and green tag, and to make a healthy choice. As the study authors note, most efforts to encourage healthy food purchases focus on nutrition labeling on the package, which is useful — assuming that people read and understand the information.

Colored stickers, on the other hand, are a lot easier to understand than fat grams and serving sizes. As Thorndike and her colleagues write, “Any of these strategies could be easily translated to other food service environments” — not just on a child’s playground.