Well, those cards can be a separate area of collecting unto themselves. Color isn't always the main thing. And there are a few other colors besides black or green.
Look at the wording of different ones. Especially in the amount of the cost of postage. Some will have printing like the one above, and other will have an actual "3 cents" or "5 cents" printing.
To a collector who wants to get the actual correct one will need to know the dating of such stuff. And of course,, depending upon the vintage, the type, etc all can affect values.
I know in my display a few years ago, with the OM .45 Blackhawks, there was a transition of those cards. If I remember correctly,, the early ones, had a "5 cent" postage,, then just a year or so later, they went to the non-denomination type like above. All between 1971 & 1973.
So the green ones & the black ones like shown would be from that era or later.
Then you have the fact that sometimes you find 2/3rds of the 3 part card. Lots to discuss here.