The Cataloging Committee of the Souvenir Card Collectors Society has gone around and around on this topic, so I thought I'd go public and see what other collectors have to say. Over the years a number of cards have been identified with distinct color varieties. These are not occasional errors that might have been caused by over- or under-inking, but something that occurred on a significant number of cards. I'm going to provide comparative scans, without going into details about how these varieties may have occurred.
First, B-1 (Sandipex) and B-3 (Fresno), both printed in 1969. I'm grouping these together because, apart from the differences in text on the cards, the three City of Washington vignettes are identical and were almost certainly printed at the same time, with the text for each show added later. There is a pink and no-pink variety for each of these cards.
The color difference on PS-8 (Nat. Postal Forum '72) is more subtle, but compared side by side, there is a light blue and dark blue variety.
Two varieties of green appear on B-68, the Compex 84 card. There are two distinct colors here, when closely compared – a forest green and a lime green.
Lastly, on B-250, the 2000 card of the Bureau's savings bond series, two signatures of U.S. Treasurer Mary Ellen Withrow appear: a smaller official signature in brown along the bottom, and a larger autograph above it. The larger signature can be found in both blue and black.
Scott lists all but the last souvenir card, as SC5, NSC2, SC23, and SC94. If Scott gives individual catalog numbers to color varieties on stamps, would that go for cards, as well? And if so, how would one go about presenting this and to whom?