Welcome to the hobby!  :)  If you ever set up an
online gallery, let us know -- I'm always jonesing
to see more Gundam cels, and from your post above,
it seems that we share that taste.  *grin*

The back-painting technique is used in cels which
are dramatically lit with harsh front lighting or
any kind of rear lighting.  It's an extra layer of
protection to make sure that no pinpoints of light
or splotchy streaks show through possible flaws or
thin spots in the painting.  They're usually
called "backlit" cels.

Congrats on your Duo, by the way -- what a great
cel!  :)