This is my understanding, and may not be accurate, so please, take with a grain of salt:
The red board is the same as the green board - it is only a difference in color. However, the deployment of the red boards coincided with a few "tweeks" that improved the sound. These were basically different brands for some of the components (better parts, if you will). Thus, generally speaking, red board amps are newer than green board amps, and have a few new tweaks.
Again, this is my "understanding", and may not be accurate.