Artistic, stylistic, or separation for sequels sake, we may never know. For twenty five years Ghostbusters II has had the iconic shampoo looking pink slime. You may not have been aware that in promotional material before and after Ghostbusters II was and had been in theaters the famed river of slime was in fact green. I believe the real historical “mood slime” was always a pinkish color. That may still be classified and I’m sure I can’t discuss it publicly.
Observe Dan Aykroyd as Dr. Ray Stantz discovering a river of green slime:
YouTube Source: P2P Productions
Comics Source: Ecto-Containment