To enamel both sides, you’ll need some sort of holding jig that
touches (ideally) only the (metal) edges of the piece. 3-prong
trivets are commonly used, but some shapes require more clever
You do want to enamel both sides of the metal, ideally with a
roughly similar thickness of glass; this balances the stresses that
could otherwise make the enamel crack and flake off.
For counter-enamel- I’m personally partial to cobalt blue, black, or
a dark gray IF the backside is going to be seen by anyone. If not, I
tend to use the leftover enamels that I get from washing or sifting
(if they get contaminated) etc., or odd lots of enamels that i don’t
have another use for, or similar. When I’ve purchased “counter
enamel”, it’s looked to me like a mix of the stuff that for one
reason or another didn’t sell well. So- in my experience, counter-
enamel has no especial properties of its own, but it’s a good way to
use waste rather than discarding it.
I hope this helps!