[Joeztech]

Based on over 50 years in the business, I would say that the blue is not an issue, but violet is an issue. Violet is a true indication of gas. You will see many beam power output tubes that have a blue glow inside. Some of it is due to florescing of the glass envelope, and some may due to the electron stream deep within the tube interacting with a small amount of stray air still in the tube. It is not possible to get all air molecules or atoms out of a tube. If the amount becomes large enough to be an issue, that is when it turns violet. There is even one type of gas rectifier that operates with a gas inside, an OZ4. Some of them are glass envelope, although most are metal encased. If you find a glass one and watch it in operation you will see it glows violet. I would not advise watching it very long though.

Joe