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. This is usually observed on the interior of the plate, on the surface of the mica spacers, or on the inside of the glass envelope. It is normal and does not indicate a tube failure.
Many people feel it improves the appearance of the tube while in operation.
Occasionally a tube will develop a small leak. When air gets into the tube, AND when the high plate voltage is applied, the air molecules can ionize. These tubes glow a strong blue-purple color in normal use. They turn AC power into DC to run the other tubes. These tubes rely on ionized gas to control a voltage tightly, and normally glow either blue-purple or pink when in normal operation.