In general a current is going to be applied whether it is black or white. This is because to produce black, it has to send a current to align the crystals. Since there is no "black" subpixel, it has to send a current through all of them. The same with white. In general, there is going to be a red, green and blue subpixel. Some LCDs have one red, two green and two blue, cause those colors are harder to produce vibrantly.
But that's just theory. Practically, there is more to power consumption that subpixel. More importantly are factors like the efficiency of the power inverter, battery, etc etc. Turning off power to a "cell" isn't like shutting off a gate and the electrons just stand still. Rather, the electrons are usually bled off in heat. So the power is used one way or another in most cases.
This is why I can safely say you will see no difference in power consumption between different color backgrounds.
You are overthinking this man. Your priorities should be else where. Want to save energy? Learn to speed read. Increase your typing speed. Get Autokey and define text snippets. Play a game of memory to sharpen your mental skills. These are the skills that will save you tons of energy.
AFAIK, there are no tablet PCs that currently utilize OLED. I so wish I was wrong here. – surfasb – 2011-07-23T12:32:52.540
@surfasb: I haven't seen one myself. I made reference to OLED simply as an attempt to provide relevant and quality answer to the OP. :) – Larssend – 2011-07-23T12:35:51.673
@Larssend : and the OP is thankfull for your answer, having hard time deciding between two good answers that cover different sides of the issue. I am always gratefull for extra information. – Arjang – 2011-07-23T12:43:24.367
No doubt OLEDs are the future. Personally, I'm still waiting for my teleporter. . . – surfasb – 2011-07-23T18:47:56.817