That's what the people from Blackle think. According to them, a monitor uses more energy when it is displaying a white screen, so if the screen in your computer is showing a dark website, you are actually saving energy. So if the original version of Google was black, it is believed that we could all save up to 750 Megawatt-hours a year.
Critics argue, on the other hand, that the site is difficult to read, and that the energy you can save is less than the site claims. For a more detailed discusssion you can go to the wikipedia article on Blacle
What do you think? Do you think it is a useful initiative? Would you use this search engine?