It is a printing issue of sorts.
To make the backs easier Topps printed the green area slightly larger than the space left for it in the black layer. If the black isn't completely solid, the underlying green makes it appear darker where the green is under the black.
That's a pretty standard practice, as it makes lining the colors up a lot less critical.
Another possibility that I'll have to look at on my 69 football is that the green was sometimes printed on top of the black instead of under it. If it is the cartoon and number should look lighter than the borders. I've seen a similar thing on the front of some 81 fleer star stickers, but not for sure on anything else although it's possible.
Steve B
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