Go to Slateport City. Talk to the Chairman in the Pokémon Fan Club. If your lead Pokémon has maximum coolness, he will give you a Red Scarf. If your lead Pokémon has maximum beauty, he will give you a Blue Scarf. If your lead Pokémon has maximum cuteness, he will give you a Pink Scarf. If your lead Pokémon has maximum smartness, he will give you a Green Scarf. If your lead Pokémon has maximum toughness, he will give you a Yellow Scarf.

Scarf	How to unlock
Blue Scarf	Have maximum beauty on lead Pokémon.
Green Scarf	Have maximum smartness on lead Pokémon.
Pink Scarf	Have maximum cuteness on lead Pokémon.
Red Scarf	Have maximum coolness on lead Pokémon.
Yellow Scarf	Have maximum toughness on lead Pokémon.