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

[G1911]

Here's another kind of color change that occurs naturally, without need for a card doctor.

These are C52's, which does not have a stock variation. The card on the left bears evidence of having been affixed to something, but appears normal, with the light cream stock and clearly black ink. The card in the middle is heavily toned, its back ink starting to lighten, but still mostly black. The card on the right is heavily toned, and its back is no longer black, but an almost teal green.

The cards in middle and right were from a very old (possibly original) outside the hobby collection, stored in an album for at least several decades until I acquired it, where cards were slotted in between two sides of thick paper. These aren't rare teal backs or a different stock, it's the back changing tone and ink color from its very long contact with the album pages. This is most commonly seen on Canadian cards, where this particular type of album storage was clearly more popular, but it is encountered on T cards as well. I hope Steve can explain why exactly this is so, because I can only speculate.

If an odd colored back is also toned, beware. It's almost never actually a rare printing error.