Parallel vs. variation
 

How would you differentiate the definitions of parallels vs variations? Is the 1956 Topps white/gray back a parallel or a variation?

Parallels are usually reserved for an intentional variation on all cards in a complete set. Gray backs are only on certain series, right? And would paper stock be considered intentional back then.
I would not consider these to be a parallel, myself.

Gray backs were a variation, created out of necessity by using two different types of paper, probably because they ran out of the first one.

Parallels were created on purpose as a way to sell more cards