I have the same question for cards #84 Ed McCaffrey and #411 Al Wilson. I was entering a small group of Broncos players I found together - the McCaffrey and Wilson cards clearly have purple coloring on the back (top) while the other Bronco cards are blue.
I don't have enough common cards/other RCs to compare to. The scanned McCaffrey image in the set CL does look a bit purple, but the Wilson image is clearly blue on the back - mine is purple.
These may be as simple color variations mentioned for other sets printed in different locations, but just thought I would bring it up since this question was out there. Attached is a scan of the Broncos cards found together.
Thanks!