For long term storage it’d be metal or glass. For medium term storage WAXED paper cups. Under no circumstances is plastic really good, as the ink solvents will outgas through the plastic, leaving the ink as a skin on the outside of the cup. Plastic is good for a day or two, but for more than that, use something else.
I use small rigid business card boxes. They stack nicely on shelves and you can write pantone number + date on the end. Then i smudge some ink on the top edge.