Hmmm. I'm not convinced. IIRC, vacuum fluorescent displays have a hot wire cathode (thin, hard to spot) at the front, a mesh electrode near the back (anode?) and targets with greyish paint on them at the back. These targets usually glow blue-greenish when hit by electrons. But it is the paint that glows. Gas discharge has the cathode suspended in the gas, and it is the gas around it that glows, not the surface of the cathode. I have never seen a blue one, but I imagine argon is more likely than mercury vapour (which radiates mostly UV). Philip.