From a softcore German Porn movie of the 70's, Maedchen die am Wege liegen, two ladies in swimnsuits & blue swim caps wallow in brown batter-like mud in front of an audience.
Google suggests a way to find it. The mud fun starts at 1:03 in. Changes from a traditional match straight up into a grope and squeeze / public fingerbang, too. Not bad for the 70s.
Edit: HAHAHAHAAAA! Holy shit. Dieter Meier, who's the main male lead, eneded up in the techno-pop out fit Yello. You've heard their biggest song:
Love you, too
