colors for battery charge

It's redundant eye candy. The color encodes the same state-of-charge information as does the number of bars.

 

If battery is completely full it will show all green. But if you look very closely at the full geen you will see it is still missing one green bar at the top of the screen. One small bar. If that bar is filled too thats a very rare sight and unless you go down a very long hill you will never see all green bars. I have owned my 07 since new and although see all green alot I have never seen the one last small bar at the top green. Peeps who travel big hills see it alot apparently.
After that mostly blue bars which is the normal range of battery level.
When the battery si depleted all bars go away and you will see one or 2 purple bars at the bottom. That will happen quite reqularly if you do alot of in town stop and go driving and sit in stop an go traffic alot or drive throughs with ac and radio on. Not to worry. When one purple bar it will force the engine to come on and re-charge the battery.
Btw, the charge is software controled so even if the mfd reflects green the hybrid battery is still only charged to 80% or so. This is so the battery doesn not get overcharged and heated up.
Same with the one purple bar. It does not let the Hybrid battery completely deplete.
Be aware with the one purple bar although not an uncommon site the car will run with greatly reduced power (no battery/generator participation) but the car should charge up to one or 2 blue bars very quickly within one mile.
You will feel and immed. lost of power at one purple bar. Take it easy on the car and loaf along till you can charge the battery back up. Demanding speed and power with one purple bar is very hard on the Inverter/transmission.