if the color of websites pages is black then a lot of energy is save , as a large no of pc's are use world wide.
Displaying black on CRT monitors saves energy. However it actually uses more energy to display black than white on LCDs. CRTs are old-style monitors that are thick and look like old TVs. LCDs are currently much more popular and are thin.
Reference:
http://www.scientificamerican.com/article.cfm?id=fact-or-fiction-black-is