“Evergreen” is the current production version of the Librem 5 (hardware).
“Amber” and “Byzantium” (and soon “Crimson”) are iterations of the operating system.
The flashing process is precisely to replace the operating system, either to go from an earlier to later version, or to reflash the existing version.
EDIT: Notice the theme: a - b - c.
Amber - Byzantium - Crimson (names of somewhat exotic colors in the art world)
Birch - Chestnut - Dogwood - Evergreen - and someday, Fir (names of trees)
Evergreen was the first “finished” commercial production batch; birch through dogwood were commercial releases, but still somewhat “beta.” (further edited for clarity)