On CRTs, I don't think colour makes a difference. In theory though, black will require the gun to fire less energy, and thus use less power. However, I don't think it actually saves any energy on modern displays.
LCDs, on the other hand, could use more electricity displaying black. The monitor has to use more energy to prevent light from passing through that pixel than it would to do white, since it can let more light through a white pixel. So it may actually waste electricity.
Overall, any impact it has on your power usage would be extremely minimal.
Funny how the search bar in FF for Darkoogle still works though... hmm...
I have come to like the black and white. Well let me hope they don't kill it completely. Even if it doesn't save power, still let us use it ;p
I believe the impact on using Google with a white or black background is negligable, because it's not as though anyone sits there looking at it for hours and hours on end. Utilising the power saving facilities offered, such as turning the monitor and HDD's off after 10 minutes (*) etc would make more of a difference, as would turning everything off properly if you're not going to be using it for a while.
Also, things like mobile/cell phone chargers never really turn off and constantly use electricity (think about it... there's no off switch!), and I found that my speakers, even though on the speaker there's a volume dial that appears to click indicating they're off, actually never are, unless I use the switch on the back of the sub.
* - Problem with the HDD power off timer is that there's usually always something writing to the drive these days which means the main drive at least is usually always on. Also, having the drive constantly power cycling could end up using more power and shorten the life span of the drive.