Oxygen (O) in plasma
Oxygen is the most colourless fill here: a hard white ring with no cast either way, in a flask that goes from clear to milky as the drive comes up. Its lines run from 437 to 616 nm with nothing dominant, so the eye gets no colour to hold on to. Worth noting it closes a proper ring at all. Oxygen is electronegative enough that it could have fought the discharge the way chlorine does, and it does not.
Very hard to ionize, works only on low pressures <1 torr
