I know there is a very broad range of expertise here, hope someone can help figure this out.

I have a lot of very good quality wall to wall carpeting throughout the house.  The main floor was redone about 15 years ago, the carpet has held up very well.  But... I have the stairs wrapped and tucked and the carpet was fitted around the wrought iron railing.  The stairs go up to a landing then on to the second floor balcony with railing around.  The carpet that is on the outside of the railing has all changed color, almost a greenish hue to the medium beige carpet.  this discoloration stops at the outside of the spindles - everywhere.

The carpet is treated the  same, gets the same vacuuming and cleaning as the rest, has the same light exposure, and air exposure as it's all open.  There is nothing I can think of that would account for the discoloration. any ideas? 

There is 4 or 5 layers of paint on the railing, so no green.  Not humid either.  It's just odd.

Tried to get a decent photo, but kind of hard to see.  everything left of the spindles has a distinctively different color tone.  Will try in the daylight.

No, no pets.  I vacuum it all the same, it's just an very odd discoloration entire length of stairway and balcony.  At a loss.

Interesting idea, but same padding across the entire house, and this is the only area with that issue.

No, it's been bothering me for some time.  Was vacuuming  the other day and gave it a close look, just don't understand what is happening.

Tried another picture in daylight.  If you look to the right and along the bottom the color of the carpet is a "warm" beige but inside the cage of spindles it has a definite green hue.  This is true all along all the stairs and on the balcony.

Yes it does.  You  might have hit on it.  But I do recall them doing a fair amount of stapling.  Took one guy an entire day to wrap the stairs and balcony while the rest of the crew did the main house.