Again, this is my personal opinion, so take it or
leave it, but the statement you made on mat color
caught my eye:
"maybe sea green with purple accents for valgarv, 
light and dark blue for zel... and maybe purple 
with gold for this one."

I think it looks better when you go with either
a contrasting color and/or a color of minor
features of your cels than when you go with the
main color of your character(s).  When you go
with the main colors, then the cel seems to blend
in the background and doesn't pop out as much as
it does when you put it next to a different color.

One example I've already came across is when I
was going to frame my Chocolate cel.  I originally
put up a dark red mat, but then after putting it
up to a dark blue mat, the effect was so much
better that I had to go with it.  I've made a
quick photoshop comparison together from my scan
and adding a border to illustrate.  It doesn't
capture the full effect...the difference was
very substantial with the real cel and mat, but
should give you an idea of what I'm talking about.

If you look at the top (red) example, the red in
her hair gets blended with the mat.  So, when you
look at it, your eyes are drawn, not towards her
face, but towards the contrasting blue shirt.
However, if you look at the bottom (blue) example,
it's the opposite.  Her hair pops out and your
eyes are drawn more towards the face, which is
what you generally want.

Try some trial and error with photoshopping and
see what works for you.  All it costs you is time
and is worth it if you find a color combo that
really makes your cels pop out at you (in a good
way).