From Nookipedia, the Animal Crossing wiki
A 3-second countdown allows the player to time his or her emotion so that it can be displayed on the TPC. The player's joke book will appear on the bottom screen, if available. If the player holds down the "R" button, the photo will be printed in sepia tone, and if the player holds down the "L" button, the photo will be printed in gray scale. After a picture is taken, if the player is unsatisfied with the result, the player may retake the photo as many times as desired for free before leaving the booth.
When the player's gate is open or other players are visiting the town, the photo booth will be out of service, and the green light above the booth will turn red.