Wasn't that limited to Marvel Comics? Something that Stan Lee hated the color green and would fire everybody who used it prominently on a cover?
Nah, I heard it cited in both mainstream and genre publishing, at Arbor House, William Morrow, Tor Books, etc, and never in the context of comic books. Although, at the time I broke into publishing, it was already starting to be regarded as more superstition than fact.
I believe the blockbuster success of THE FIRM by John Grisham pretty much drove a stake through the "green covers don't sell" thing . . ..