Comrade Necro is correct, this mystery is explained by the Rising Sun.
(click for larger) STACK ON RIGHT = FADED
The stack on the left is for REFERENCE.
PHOTO 1: The STACK on the RIGHT has faded spines.
PHOTO 2: I rotated the STACK on the right 180ยบ to show you how, for most titles, the spine is "green", but, sadly, Valis IV (top) and Death Bringer (bottom) have fading on BOTH spines. Ha!
I just quickly grabbed games that were "representative" of green for the LEFT stack...I tried to get non-faded, consistent green (but it might not be perfect).
NOTE: You would SWEAR that Death Bringer was printed blue, but if you look close in bottom photo, you can see that there is the *slightest* tinge of green that fades from bottom-to-middle. The photo is not ideal (shadows), but when handling item you can see the subtleties.