This is from the FAQ on London Power's site:
Q: What does it mean when my power tubes glow blue? Should I toss them?
A: No, you don't have to toss them just yet. However, a blue glow indicates the presence of some gas in the tube, which is not a problem in most situations. If you have noticed an increase in the amount of blue, then you might want to get a new set of tubes sooner than later. Too much gas in the tube can lead to bias failures, blown fuses, and possibly blown output transformers.
WARNING!
If your tubes are glowing bright orange - that is, the grey parts inside the tube are glowing orange - TURN THE POWER OFF IMMEDIATELY! A bias failure is in progress. Usually, this is the fault of the tube itself, but can also be caused by an open grid resistor, bad socket connection, or poor solder connection. Check the bias voltage right on the tube socket with no tube plugged in. If the bias is okay, plug in a new tube.
If you want to read his whole FAQ go the the link below.
http://www.londonpower.com/faq.htm
