I've been dying for this question to be answered. In the college I go to they use cool white 4100k 32w u-bend bulbs in most areas. However some bulbs are yellow, particularly the Sylvania fbo32/841/6/eco. Other bulbs they use are the Westinghouse FB32/841/6/ecomax and the Eiko FB32/841k/6
but these bulbs have crisp white light. However the part that threw me off is there are other Sylvania ubends that are the same model number but look almost as white as the Eiko or Westinghouse but I noticed they have less dark ends than the dingy yellow ones. The Eiko and Westinghouse have no dark ends.

So my question is do fluorescent bulbs yellow as they age or is it manufacturing error? I've noticed with my own fluorescent bulbs that if you turn one bulb off but leave the other one on and 20 minutes later turn it back on, the one that stayed on is dingy yellow for about a minute or so then both bulbs look crisp white. Both 4100k bulbs.

The american energy saver T8 lamps often have an electric conductive coating inside the tube, to allow them being start on rapid-start ballasts, which is probably the cause. As the lamps you mentioned, uses triphosphors, there is no other reason for a fluorescent lamp to get a yellow body color with age (Glass don't yellows like plastic, triphosphors don't yellows as it degrades). I've never seen this with our T8 lamps (18W, 36W and 58W for 230V T12 chokes).

I've attached a picture to show you what I mean. The reddish white bulb on the left looks white and the greenish one on the right has a yellowish white color. Both 4100k T8 bulbs. They are not T12 cool white deluxe which can appear pink, I assure you these are 32w T8's. My camera quality is not the best but it gets the point across.

I've noticed it with Philips Alto F32T8 /841. They were a crisper white when new but have yellowed slightly. My room at work is full of them and they do make things look dingy. I have a lot of windows so on sunny days it's not bad, but on dark cloudy days is is quite noticeable. Haven't had EOL lamps in a few years so no new addition to compare to. Replacements will probably be non-Philips as the Philips dealer has changed brands to Sylvania and the other electrical counter still sells GE.

Some day when time permits, I might try to make a MH floor lamp and use a 4000K ceramic lamp to even colours out especially on cloudy days.

I noticed it with many Triphosphor lamps, especially T5. Looks like the Blue Phosphor component wears out faster than the others, making the color shift

I've also seen this with Sylvania, Philips, and GE T8 bulbs but not Eiko, Westinghouse etc. Although not many buildings use those brands so can't be sure since I've only seen them installed new. Does anyone here know why the color shifts and how gradual it is? Does it take a few days, months, or years?

If he meaning to the yellowing of the light color of the lamp, I've seen this with many T5s and in my Osram Dulux EL 23W/840 in my room at my father home, which is now 3500K and not 4000K.

Yes I mean the color of the light yellows. Even some 3500k lamps look more orange when they get older. Why does the color of the light shift after a period of time? Is it just ware and tare or is it the emmisive material depleting making light output not the best? Now that I look at some of my T8s the ones I installed back in January look slightly, just slightly more yellow compared to the Sylvania's I installed a couple weeks ago but not enough to notice a difference unless you look for it. But the ones I've seen are next to yellow compared to newer bulbs. Even in T12's I've seen this.

The cathode emission material has nothing to do with the light color.

The yellowing would be most likely the phosphor degradation...

I've actually never seen any fluorescent lamp turn yellow.
But yellow covers are unavoidable!

I've seen some that to a yellow color at eol, this's caused by sodium leaching out of the glass and staining the phosphor. This happens when the instant start has a spectacular burnout. I've seen some 4100k tubes look yellow in use, but phosphors look clean when off.

What sort of lamp is it that uses Sodium and Phosphor in the same lamp ?

What sort of lamp is it that uses Sodium and Phosphor in the same lamp ?

Sodium is in the glass...

But it is in a compound there, not as free metal..?

It is, but that does not prevent the soda to react with the phosphor components and by that exchange the Na ions...