1953 Bowman Color and B&W Card Backs

[jingram058]

Have seen discussion here at net54 and other forums about the many card back variations with Topps in the 1950s and 60s. I just finished a set of Bowman color raw, and I notice some of the backs of these cards are lighter in color, slightly more reddish, and some are darker and slightly more brownish gray across the entire set of 160. Looking closely with my 10x loupe, I see it is the card stock itself. I also see it in listings on eBay and other auction sites, but can't say for certain in online images where there are any number of brightness/darkness/contrast possibilities. I believe Bowman made their cards in layers like this: with the front applied or glued somehow to the card stock, and the back printed directly to the card stock, and then the sheets were trimmed. Several questions come to mind. Is this variance in card stock color common in this color set, or perhaps all Bowman cards, similar to Topps? Is it due to how these cards were handled and saved over the decades, and they lightened or darkened due to sunlight, heat, cold, humidity or lack thereof, or what have you? There is no question about authenticity here, I'm not talking about that or bringing it up; the cards are all real, no doubt about it.