The carton is white and has a dark blue strip on it that reads "Milk" in white and in all caps. The milk itself is regular milk.
Role in episode
In "The Secret Box," after SpongeBob tells Patrick some of his secrets to try and make Patrick show him what's inside his secret box, Patrick refuses, saying that while SpongeBob is an open book, he is more complicated, saying "the inner machinations of my 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 Plankton finishes the story.