Colors on Energy Monitor-what do they mean?

I have a 2007 Prius that I purchased a couple months ago. Still trying to figure it all out! I have noticed that the battery icon on the Energy screen is sometimes blue and sometimes green. I read somewhere that the green means the charge is at it's peak? Anyway, it has been staying green more than blue lately and I was wondering if that was cause for concern or celebration?

Also been trying to determine whether loss of mpg in the cold weather is due to the cold weather? I drive 4 miles to work one way and once the temperature outside dropped below 45, my mpg has dropped from 47 to 35. I'm guessing that's because the ICE is working more to warm up the car and since my trips are really short, it is using more gas on average?