On an LCD, black uses the <i>most</i> energy.  Granted, it's a miniscule difference.  The backlight is always on, and energy has to be passed through the crystals to twist them and block the light at the individual pixels, in order to produce anything other than pure white.<p>On a CRT, black is probably the greenest color.  But in both cases, I suspect the difference is so small that you wouldn't be able to measure it in pennies on your electric bill.