Yellow is simply happy. It has long been associated with energy and encouragement.
It can cheer you up on a bad day, so if you're feeling down, wearing yellow could help perk you back up.
It is also associated with intelligence and inspiration, so wearing something yellow on the day of your big exam might help you come up with some extra answers.
Whoever is fond of yellow is outgoing and enthusiastic about changes, characterized by a great sense of imagination.