The disc was made at the PDO in Blackburn, Lancs during the late 80s, early 90s. It's down to poor manufacturing rather than poor handling. There's a section on wikipedia about it (search "PDO bronzing"). You could try removing the booklet and insert to prevent further corrosion (it's the reaction with the sulphur in sleeves that causes the problem).
Hmmm...my disc was made by MPO not PDO (I knew that one company had that problem, and thought it was MPO. Apparently not).
The disc plays OK and - having had it about 20 years - assume that either it's not going to be a problem or that the disc was that colour all along, and I just hadn't noticed until I came to scan it.
Yup, I'd say you've got a case of CD Bronzing there... which is a kind of "CD Rot"
I had one a couple of years ago that I'd not looked at for many years, and once it's started turning that way, the CD is done for.
Enter the terms I mentioned in the search box (i.e: Google) for Wiki entries on it.
Basically, PDO didn't layer the plastic coating consistently early on, and where it was thin enough, and wore through making a breach, exposing the shiny layer to the air, the oxygen reacted with the foil layer, spreading throughout the disc under the plastic layer.
PDO used to replace any pressings subject to this until a couple of years ago I think, if you sent the disc and proof of purchase back to them, but I believe a kind of statute of limitations has expired (They only did this voluntarily anyway).
Oddly, it was the mix of the oxygen and whatever chemicals were used in the booklet/cover paper, in which it was in close proximity, that set it off.
If you keep it for curiosity's sake, the disc will get browner, and go completely over time.
Strange thing is, that in general, they figured that most of the CDs that would have been affected would have gone this way a long time ago, as it was early pressings that were subject to it, and any that hadn't shown any signs by now, could be considered as safe, so to have one in a "mid" state of bronzing now is unusual.
Having now come to scan this item, I don't remember the disc being that colour before. I seem to remember that there was a problem with discs pressed by MPO in the UK that they did turn brown after a while (although I can't find anything about that with a cursory online search, Could this be one such victim?