Pinkie Pie has an existential crisis while painting her room regarding the true nature of identity and the fleetingness of the self, sparked by her observations of a once-orange wall being turned pink without its consent.
It turns out that she just gets that way when she doesn't have enough sugar, and that the cure is to eat a cupcake.
Although this fic was originally written as a joke by the author ("This self-parody is dedicated to Professor Piggy, who once told me that I could write a story about Pinkie watching paint dry and weave it into an incredible psychological journey. Let's see if he was right.") it quickly became infamous within the MLP fanfiction community, even overshadowing The Games We Play, the author's other best-known work.