What's the episode with the old uniforms?

You are thinking of the most commonly asked about Real Ghostbusters episode, Citizen Ghost, which detailed the events set immediately after the Ghostbusters' battle with Gozer. The episode elaborated on how Slimer came to live with the Ghostbusters in the Firehouse, and why the Ghostbusters wore multi-coloured suits: the Ghostbusters' old tan suits had absorbed large amounts of psychokinetic energy from the battle with Gozer, and later absorbed even more energy from the Containment United when Peter forgot to burn them. The build-up of ectoplasm caused the suits to come to life, creating sinister green ghost clones of the Ghostbusters who had one goal: to destroy the Real Ghostbusters and replace them. At the episode's conclusion, Slimer was able to help defeat the ghost clones, securing his place as the Ghostbusters' mascot.