Back color doesn't matter. Basically anything that would get a number grade, up to around Ex+ (low to mid-grade, doesn't matter, my set is all over the place). Just wanted people to know that it's not a spot where I'm interested in getting slabbed grade 6 minors for 30 bucks or higher grade commons; doing this set economically.
I have extras of at least a few of yours too, but would have to check and see if they're grey backs and at least VG. My '56 doubles have been bottom priority to get organized, but this would be an excuse to finally do it.