I recently fitted 16 of the 4GB Green drives into a backup server which runs Linux. I used the WD idle3ctl program to check and turn off idling, which was set to 80 seconds if I remember correctly, definitely much longer than the 8s that was once the factory settings on these drives.
Thanks for the information, this is really good to know. It seems WD is shuffling their pack regarding the firmware of Reds and Greens - might be removing overlap, might be something else...
Regarding 2.5" drives, I have no idea. I've never used them in desktop builds. In laptops I've found at least one Blue to be noisier than my current Green (WD10EARS-00Y5B1), but this was years ago.