No, you'll be able to get those colors with deco.
No. The whole reason I came up with FLMT was because FILT colors are too easy to accidentally change. BOMB, DEST, EMBR, and DMG already glow with their deco color, and I don't want to have red (or blue, depending on how the color scale would be implemented) FLMT melt my wires and destroy my light show. And, if FLMT is to change colors mainly based on temperature, then it would have to be almost indestructible, and FLMT is also meant to be a destructible version of INST.
Filament should react with MERC as well turning it into a cold plasma that is the same color as the filament.
I think it would be better to make it turn the mercury into cold plasma and electrons, then add another element (PHOS) that glows when hit by radiation.
If one were to plonk down a huge block of "ETRD", it wouldn't be an electrode anymore, would it? Of course, FLMT could always be renamed to LITE if needed.