A search will likely turn up lots of posts on this topic. In a nutshell, the different colors simply provide a quick visual indicator of what level of charge the batteries are holding. My car stays in the blue or green sections almost all the time. There is at least one other color below blue, but it's been so long since I've seen it that I can't remember what it is.
If you have a 2004, there are a total of 8 bars. If the charge is 1-2 bars, the bars appear red/purple (depending on your definition of the color.) If it's 3-6 bars, it appears blue. if it's 7-8 bars, it's green. The color is just a 'quick-glance' way of determining the approximate charge. (i.e. Green is full, red is empty, blue is 'normal'.)
I haven't seen a 'yellowish-green'... Then again, I don't know about the 2001-2003. If you see the different color at the same charge, it could be the LCD warming up.