Quoted from KevinCPR:
Exactly. Any personal anecdotes related to other machines from the same era holding their colors, has nothing to do with what may happen to a particular individual title on it's own. Heck, even if the Bally games produced just before Fathom, and right after Fathom never color shifted one bit. Not the point. That isn't evidence Fathom didn't. Because Fathom certainly DID. And badly.
I think it has to do with the specific blue pigments at the time, used in those particular blue mixes. Right at the same time, Stern's SEA WITCH (copycat of Fathom) suffered the *exact same* color shifts to it's blues. Widely seen today, almost all Sea Witch's suffer from it. You can go back to the Stern flyer on those too. Same blue themed starting point. Same issue. Same 40 years of color shifts to gross greens, seafoams, and teals on the playfield - while the plastics, glass, cabinet, etc all essentially retained their blues. However, there are some decent survived Sea Witch playfield examples today that still lean to the blue scheme. Even looking at IPDB pictures, or Googling "Stern Sea Witch playfield" the degree of color shifts is far and wide.
I have an original Fathom I bought in 1982. The playfield is pretty close to the same blue that the last CPR repro has. Same goes for the cabinet colors. Everyone paints the cabs,copying colors that are aged and faded. Fathom’s color scheme was a dark blue with a baby blue and a neon green on the cab.