The cards appear to have been issued in two styles - with an overall amber tone, and a lighter, brighter version. According to the Russian seller, the amber variety are from 1956, while the brighter versions are from 1962. In the set I have pictured above, the Emerald City card (#12) is of the brighter variety - it's quite a difference!
*After posting the above, I learned a bit more from David Maxine about this series. It seems more likely that the varying colors of the cards may be due to an aging varnish finish on the cards, rather than different printing choices and dates. Also, the 1956 date doesn't seem likely as these illustrations weren't used until 1959!*