The uniforms worn during the Gozer battle had too much ectoplasmic radiation and needed to be destroyed. Peter (ironically?) was suppose to take care of this, but didn’t. By the time he realized he still needed to the ectoplasmic uniforms had absorbed more PKE from the new Ectocontainment Unit because of a small leak. The PKE imprinted on the Ghostbusters uniforms, becoming green, ghostly versions of The Real Ghostbusters. They even copied the proton packs!