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.