Normal color, don't worry about it. Subject has been discussed in detail on http://www.audioasylum.com/scripts/search.pl
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Yu: What type of tubes are we discussing? A Blue/Purple glow @ the top of the glass globe is usually OK on larger power tubes (I have often found them to be the best sounding), but if it extends lower and seems to cloak the horizontal spacers and/or plates, then you may have a slight leak in the tube which will considerably shorten its life. It is unlikely that "all" of the output tubes would develop leaks unless someone attempted to improperly straighten the pins on them.
Yul: The blue is very common with rectifier tubes and also output tubes if it is limited to the top of the output tube, just above the vertical plates. If the glow seems to scatter below this area and/or fills the glass globe then I would suspect a leaky tube which should be checked ASAP. I am not familiar with 6SN7's (never owned anything which used them) but perhaps some of our BAT, Blue Circle, et cetera, owners will respond. You might also try switching the glowing tubes around(while keeping track of their previous location) to see if the glow follows them. Sometimes another part of the amp (or even plain metal) can stimulate this glow just by being in close proximity to perp tube. I read about this once not really understanding the explanation, but recall that it was not a big deal (anything to worry about). Maybe give the manufacturer a call as well?