In "The Secret Box," the carton is white and has a dark blue stripe on it that reads "milk" in white and in all caps. The milk itself is regular milk.
In "Plankton's Good Eye," the carton is white and has red writing on it that reads "milk" in all caps. The milk itself is once again regular milk.
Role in episode
In "The Secret Box," after SpongeBob tells Patrick some of his secrets to try and make him show him what's inside his secret box, the latter refuses, saying that while the former is an open book, he is more complicated, adding that "the inner machinations of [his] mind are an enigma." Patrick then imagines a live-action carton of milk spilling itself.
In "Plankton's Good Eye," Plankton tells a story to SpongeBob in hope of making him cry. Near the end of the story, Plankton spills a carton of milk that SpongeBob placed, making him cry when he finishes the story.