I could be mistaken, but it only appears to be differences in the heater element. Since tube manufacture is not an exact science, it depends on how much of the filament is showing at the end of the cathode sleeve. I have seen variations of the heater glow in the same tubes (including preamp tubes) and it is little or no concern. If it was a JJ power tube, I would say it would have a short life, my experience with JJ power tubes was short and they are no longer an option for me. Also what you see on one side of the tube may not be what can be seen on the other side. Sometimes, the seam on the plate may be showing the onset of red, depends on the volume settings and will only become apparent during operation and not on standby. Your pictures look normal. I have also seen some tubes with little or no heater glow (typically preamp tubes) but you need to look into the slots on the plate to notice.
In complete darkness, you may see a blue hue in the tube which is normal. If only three of the tubes are blue and the subject only has header glow it may not be passing any current. No blue glow is not an indication the tube is bad. However, while observing the tubes, does the yellow glow increase while playing? If it gets brighter and dims with no signal it may be time to replace. Often may be the case with a screen resistor issue but doubtful. Does the amp sound harsh or overly compressed? I have had a tube cut out once and the amp went out of balance and sounded super compressed. You could swap the inner tubes with the outer tubes and observe. Keep the matched pairs matched. Outer tubes are one pair, inner tubes are the other pair. Note: if you are operating at half power, only two tubes will have the blue glow and the others will be off. If any of them glow purple or pink that would indicate a vacuum leak and should be replaced. Blue normal, purple or pink no good. Red definitely bad but usually on the plate and not fluorescence on the glass tube.
How old are the tubes (perhaps the better question would be how many months of use are on the tubes) ? If you have more than a year of use on the tubes, good time to update to new tubes. One a power tube reaches end of its life, typical symptom usually is lack of character, sudden change in tone, loss of bass or treble, uncontrollable feedback, and could eventually red plate or stop flowing current.
