There are three kinds of blue glows in tubes, all fun to watch after a beer or three.
If the glass envelope seems to be glowing blue in patches, that's fluorescence caused by electrons that somehow missed the plate and hit the glass. This seems to be at its strongest in brand new tubes, and gradually dims out as the glass gets sputtered with a thin film of cathode material. But even once it's gone, the tube should still have plenty of life left.
In my experience it disappears in the first 10-100 hours of use, but a good tube should last several thousand hours. I have a couple of Sovtek 6550s that glowed strongly until I left my amp in a rehearsal room we shared with a few other bands. When I got it back, the tubes didn't have that nice blue glow on the glass any more, but they still gave full power. That was a year ago and they're still working.
Sometimes moving the tube to a different amp will make the glow appear again. The different voltages mean that the electrons hit in different places that are still fresh.
If you see bluish-white stripes on the inside of the plate, same thing. Horizontal zebra stripes for a pentode, and one vertical line on each side for a beam tetrode. I've seen these even in old, worn-out tubes. They're brightest at the beginning of a run when the plate is cold, and fade out as it heats up.
A cloud of glowing gas inside the tube is always bad news, it means you need to junk it before it shorts out and damages your amp. I've got some really old Sovteks that do this, and it looks quite different to the other two kinds of glow.
Maybe you should swap the two tube pairs around every so often, so they wear evenly.