I've always wondered what the CCT of a fluorescent tube is that gives you that yellow color. I know it isn't the refractor yellowing (see image); maybe it's just the tube aging since 4100K is the most common CCT in North America, which is much whiter than the yellow fluorescent tubes. So, what is it?

If the diffuser isn’t yellowed, then it could be the aging of the phosphor, especially in triphosphor lamps. Over time, the blue phosphor will diminish in brightness faster than the green and red phosphors, which will start to push the lamp towards a warmer color temperature regardless of what the intended color was. Even on 3000K lamps, some will start to look 2700K-ish over time.

In this particular instance, this looks like well-used 3500K or 4100K lamps that have aged to look more 3000K-ish but with a pronounced yellowish tint.

These are probably 2700K or 3000K tubes. Phosphor deterioration is not as much of a problem on fluorescent tubes because they are not driven as hard. LEDs do noticeably degrade in color, though.