The green computing movement demands that all computer users shed the energy-wasting practices to which they've grown accustomed—so you decide that you're going to power down your PC at night, invest in an Energy Star–approved laptop, and only visit Web pages that eschew white space in favor of ostensibly more energy-efficient black backgrounds.

Before you tune out and turn off, you should know that black isn't necessarily the new green. Because computer monitors come in a variety of shapes and sizes, and not all monitors create black and white the same way, there's no proof that, on the whole, increased usage of black images would save more energy than the continued use of white ones. In fact in newer liquid-crystal display, or LCD, monitors white is actually slightly more energy efficient than black.

The notion that black screens save electricity certainly makes sense when you're talking about cathode-ray tube, or CRT, technology that works by moving an electron beam back and forth across the back of the screen. "The front screen is covered with red, blue and green phosphors," says Bill Schindler, vice president of electrical engineering for Panasonic Plasma Display Laboratory of America. To produce white, the electron beam is directed at the phosphors. However, "when the screen is black, you don't have to fire the beam," he adds.

CRT monitors, which until a few years ago were the predominant models among PC users, consume more power when a computer screen is white. To confirm this, Schindler measured the energy output of an 18-inch (45.7-centimeter) CRT monitor and found it used 102 watts when the screen was white but only 79 watts when the display was black.

This is not the case, however, with LCD monitors, which have no phosphors and represent the lion's share of every new monitored purchased in the developed world, including those used by laptops. Instead, LCD displays rely on an array of thin-tube fluorescent bulbs that provide a constant source of light to create a white screen. To make it black, LCDs rely on a diffuser to block this light. As a result, LCDs use more energy than CRTs to display a black screen. Measuring a 17-inch (43-centimeter) LCD monitor, Schindler found that white required 22.6 watts, while black came in a tad higher at 23.2 watts. With a 20-inch (50.8-centimeter) LCD, black required 6 percent more energy than white.

One of the most visible manifestations of the belief that black screens save energy is Blackle, an online search engine whose Web site is cast almost entirely in black. Created by Heap Media, Blackle exists "to remind people of the need to take small steps every day to save energy," says Blackle founder Toby Heap, who launched the site in January. "I do not expect the energy savings from Blackle to change the world on their own, but the point of Blackle is that every little bit counts."

One of the key arguments in favor of black screens is a 2002 research study produced by Lawrence Berkeley National Laboratory entitled "Energy Use and Power Levels in New Monitors and Personal Computers." The report indicates that "a given monitor requires more power to display a white (or light) screen than a black (or dark) screen." Indeed, that study reports that black screens consistently require less energy than white screens, regardless of whether the monitor is a CRT or LCD.

"It depends on the resting state of the LCD as to whether they require energy to stop light or to allow light to pass through," Heap explains. "This is why screen tests show that some CCFL (cold cathode fluorescent lamp) LCD screens save energy and some use a bit more. All of the scientific test data we have come across shows a slight saving on black LCD screens, which suggests that the rest state on many LCD screens does not allow light through." Heap also points out that a large number of Blackle users come from India and South America, where CRTs are still commonly sold.

Even though Google isn't tied to Blackle other than powering its search engine, Google green energy czar Bill Weihl in August posted a blog disputing the notion of black as the new green. "We applaud the spirit of the idea, but our own analysis as well as that of others shows that making the Google homepage black will not reduce energy consumption," he wrote. "To the contrary, on flat-panel monitors (already estimated to be 75 percent of the market), displaying black may actually increase energy usage."

New advances in LCD technologies could eventually validate the belief that black is better. Newer types of LCD include a dynamic dimming capability that changes the strength of the backlight based on the image being displayed. Heap also points out that many of the new monitor technologies such as LCDs backlit with light-emitting diodes (LED), plasma screens and organic LED screens do not have a constant backlight "so we will see larger savings with Blackle as these new monitors replace the CCFL LCDs," he says.

In the meantime, the world is evenly split between CRT and LCD monitors, totaling roughly 405 million and 401 million respectively in 2007, according to iSuppli data. So if you're still toiling away in front of a hefty CRT monitor that takes up three-quarters of your desk, then black screens will save you some energy. For those who've graduated to thinner LCD models, black screens are actually sucking up more energy then their white counterparts.