A google employee (Jeff Sharkey) has posted an interesting post - testing how using different color filters in Android (on the Google Nexus One) to test the OLED's battery life. He use SurfaceFlinger (Android's low-level window composer) and created several options: from red-only to full-color.

Google Nexus-One color filters photo

Using just red halves the power consumption, and using red+green results in 25% better consumption. Jeff shares his patch online , so you can download and see for yourself. The phone remains usable, so this is an interesting way to save power... Jeff also posted a short video showing how the phone behaves using the different filters: