The blindingly bright shade of green on a new Toyota Prius has a reason other than a designer's whim: The paint is also designed to keep the car cooler, so drivers can save energy by avoiding the air conditioner on hot days.
"We chose lime green considering two factors: design needs and heat-control performance," says Toyota spokesman Takashi Ogawa. "SRP [solar reflective paint] works well with light colors. The closer to white the color is, the better SRP works."
A white car can reflect as much as 60%-70% of heat from the sun—one reason, maybe, that white has been the most popular car color for several years in the U.S., especially in hotter cities. Silver cars also bounce sunlight away; in a 2011 study, researchers found that a silver car with a solar reflective coating could be around 11 degrees cooler inside than a black car parked next to it on a hot day. But the challenge has been getting the same effect in other colors.
Toyota's "Thermo-Tect" lime green coating uses a proprietary mix of paint, filled with titanium dioxide particles that help reflect the light. It was hard to get the mix right—the bigger the flakes of titanium dioxide, the more light bounces off, but it also makes it more difficult to get the coating to stick to the car.
The new coating can affect the temperature by nine degrees, Toyota says. The company doesn't say how much energy that can save, though the 2011 study found that the AC ran 13% less in the solar-coated car. In separate research, scientists calculated that if the temperature of all cars in the U.S. was reduced by five degrees, it could trim CO2 emissions by millions of metric tons a year.
The paint could make even more of a difference if combined with reflective windows. California briefly considered requiring all cars to add window coatings, estimating that it would prevent 1 million tons of climate pollution on its own, though the state later changed its mind.
For now, Toyota's custom lime green paint is only available in Japan.