Plus, it felt like the person on BGG assumed the difference in colour of the backs was directly related to the artwork. Once they were seeing C2 and C3, all differences seemed to be assumed to be related to that.
As Whaleyland said, there can be differences in the tile backs in terms of colour, but this has been seem before within artstyle releases. Different print runs will get that.
And sometimes it might be a bigger difference, but that doesn't mean the whole artwork is incompatible.
If it is really really bad, it can also be a misprint.
I wonder how they would feel about the 50 shades of green that exist in the game. This is more prominent in C1, but within C2&C3 it can also be seen