I just came across an interesting post about energy savings based on the colour of a webpage (specifically, Google – but the idea applies everywhere): A black Google start screen? | Wired Gecko.
I've found other posts dating back to May of this year (The Numbers Guy) on the subject, so it's really not a new idea.
The largest part of my doubt is the question of technology. In a CRT, it does indeed use more energy to display bright colours than dark (ref. DOE Energy Star Desktop Information). A CRT produces bright colours by directing an energy beam at the front of the screen; more brightness == more energy used.