Red to Orange in c. 1910 T and E cards.

[steve B]

There's a lot going on there. A few different things all at once.

The typical scrapbook or album years ago had pages that weren't acid free if anything they were pretty acidic.
Products of the fibers in the wood pulp breaking down over time. Some huge percentage of books produced around the 1890's -1920's are mostly doomed if they don't get deacidified. So much so that LOC tried building a machine to deacidify books in bulk.
That acid makes the paper turn brown and brittle. It can and will transfer to paper that's held against it, even staining acid free paper.
I don't know for sure if those cards are also acidic, or if that's just staining from the acidic album pages. It could be either. The T220 and T218 cardstock is actually pretty good, I haven't looked at it super closely, but some older paper - pre optical whiteners had a high rag content. So It's got fewer (Or no) wood fibers, instead having flax, wool, cotton and maybe silk. Once bleached, it's hard to tell. I believe at least some T card cardstock is at least as acid free as the stuff you can get in the craft store.

The second thing is related to the third.

The inks could be darker or lighter depending on how much colorant was used. What the hardening agent/carrier is affects it too.
Black was usually carbon, which won't fade, and isn't really probe to chemical changes either. But it could also be a chemical dye, and those are changeable. My kid chemistry set had a way to make black ink, then to change that ink to blue. And the cool part was also being able to change it back to black.
So it may be the acid in the paper changing the ink
OR
It may be the ink being slightly translucent, and the color you "see" is the paper color darkened by the overlaid ink. Everyone will see this differently. I see the right one as a gray with maybe a bit of overtones of blue or yellow. If you're seeing teal, it may be that you are better at seeing yellow shades, and that the ink I see as Black/gray is actually a very dark blue. Mixed with a bit of enhanced yellow... might = teal. So we could see a color totally differently, and both be "right" (And yes, that's effin confusing!)

And there's a good lead in to the third possibility.
Since we all perceive color differently, that color difference could be from the "black" being seen against a yellowish background.
That sort of thing is the underlying thing in modern cmyk printing, the four colors mix in perception to produce ALL colors.
I was going to write up a thing about this for more modern cards, where two of the same card have slightly different shades in the sky it's often a slight registration problem.

All this could be put to rest by identifying the ink composition etc. But it would take access to some really involved equipment.