Just thought you might like to know that I am not just spouting off to run you down. Definitely hope you take this as intended, as it NOT critcism. I have decided to see what the web has to offer on the subject. A tight tube has no gas to ionize in the first place. Glass tubes have visible glow inside them.
Most audio types use oxide-coated cathodes, which glow a cheery warm orange color.
All of these are normal effects. This is a very deep blue color. It can appear wherever the electrons from the cathode can strike a solid object. It is caused by minor impurities, such as cobalt, in the object. The fast-moving electrons strike the impurity molecules, excite them, and produce photons of light of a characteristic color.