I usually have the opposite approach to yours.
Instead of trying to match the primary colors of
the cel, I try to choose another color.  The
reason is that if you match the colors, the cel
tends to "bleed" into the matting.  If you choose
a different color, it can "pop" the cel out more
towards you.  Taking your mock-up, I just changed
the matting into a dark reddish-brown and now the
cel seems more prominent to me.  Also, depending
on where you're displaying it, you might can
choose the matting to match your decor.  Just a
different approach.  Take it or leave it.  I do
love your idea of using the trim to accent fine
points and the gold-ish trim does wonders.  I'll
have to try something similar on my next cel.
Thanks for sharing.