My 300B s glow differently ????

My amp uses 4 300B tubes. With the lights out they glow a very pretty shade of blue. I understand that this is normal. However, one of them does look the same as the other three. The glass on three of them glows uniformly all the way around and top to bottom. The other one is darker in the middle. Is this a problem or am I being too obsessive? I'd like to find out now while the tubes are still under warranty. Thanks.