I found it! I was scrolling through horror category so I remembered lol. It was this. A film from 1971, so you weren't far from the year it was made. :D

Ah yeah I only have a tray with no air tight lid, and the temporary lid only extends it for 48h maybe. I agree with the paint consistency though, if I let it stay there for too long they become too runny and I wouldn't want it on my paper sketchbook. XD I definitely need to buy an air tight lid. That's pretty smart tho, clean them all at the same time! :D

I recently just started putting my wet gouache from the tube to watercolor pans (empty palette) and dry them just so I could just use that when I'm lazy. Acrylics and gouache are so different tho so will need a learning curve for it but still good for days when I just want a quick paint.