Favorites can be set for a Sim in Create a Sim as part of their personality in The Sims 3. There are three types of favorites: food, music, and color. Sims will tend to choose their favorites when they can.
In The Sims 3, Sims have a favorite color. The purpose of this is depending on the light colors, room colors, fireplace colors, and the color clothes they wear. When they are in a room with that specific color, they will have a better mood.
The purpose of the favorite food is apparent: Sims who eat their favorite food will clap and laugh as they eat, and might get a moodlet, and get a mood boost. Sims will choose their favorite leftovers more if told to eat. With Late Night or Supernatural, vampires will prefer O-positive plasma, a type of blood in real life.
Showtime or patch 31 included some new vegetarian and tofu-contained recipes. These recipes are the alternatives of the existing recipes.
Sims listening to their favorite genre of music will get a mood boost.
Due to a glitch in the game, it was possible to have a Sim's favorite music be "French" without The Sims 3: World Adventures. This was fixed in game update 1.8/2.3 on 12/15/09. Subsequent expansion or stuff packs will also change a Sim's favorite music from "Custom" to anything else.