Making your screen BW won't decrease your power consumption. The backlight of your LCD is what consumes the power, and this would remain on even if you had a black desktop background. You can increase your battery life by:
1. Dimming your screen
2. Don't play music / movies or use programs that constantly access the hard drive.
3. Turn off bluetooth and wi-fi when you're not using them.
4. Quit programs when you're done, don't just close the window.