I have non-card stuff that has rectangles of brown on it from being in contact with some acidic paper for a long time. But it's not purely from the contact, it takes contact plus one bit of paper breaking down.
Put a newspaper clipping in a nice white envelope for a few years. If you can accelerate the breakdown by putting it on a sunny windowsill so much the better. Eventually the clipping will leave a brown stain.
Cardboard is basically wood pulp with some other stuff added. Grind up some sawdust in water and that's basically paper pulp. Pour it onto a screen press it flat and let it dry and you've got paper/cardboard. Wood contains lots of cellulose stuck together with lignin which breaks down over time.
http://www.ehow.com/list_6591100_cha...cardboard.html
Various grades of cardboard are bleached, or may include stuff other than wood pulp. Depending on how it's made it can be more or less acidic over time. The cheap stuff like brown cardboard boxes will get pretty acidic and brittle. Better stuff will be usually be whiter and take longer to much longer to breakdown. Unless the bleaching chemicals make it worse. (60's Topps are average ish, 20's strip cards are on pretty bad stock and I doubt they'll be collectable in 200 years, T206s are nearly acid free and should be with us for a very long time.
I checked a few 67s and they're not multi layer- at least the first series.
I also don't have any with a lot of fading. But I do have some with cardboard that's been treated differently.
69's are the most obvious for the different treatments of the cardboard, and some show more breakdown than others. Here's a pair, one with degraded stock that's been left raw, the other has had a layer of white printed before any of the pink or black
Another thing that could cause the fading is if they're from dealer inventories that got long exposure to light, like a flea market dealer or something like that.
Another thing that came to mind would be the reaction of the ink itself. The green may be more fadeable by the breakdown products than the yellow or pink.
Telling if it's different cardboard or just different storageis a complex thing that I just know some of the basics of, largely from other hobbies. (Stamps, old books, film*, other ephemera) I could probably figure out a lot more, but I'm not really equipped for that level of hard science and my chemistry has gotten a bit rusty since Highschool.
Steve B
*Lots of film is Cellulose acetate which breaks down giving off acetic acid - Vinegar- which can accelerate the breakdown if it's in a sealed can. The old film on Cellulose Nitrate gives off Nitric acid and when it catches fire self oxidizes making it darn hard to put out.