Answer 1

Show the man at Pastoria City (near the Great Marsh) a Pokemon with max Smartness for a Green Scarf.

Source: Experience

Answer 2

In Pastoria City, when you talk to the guy, your Pokemon must have full smartness.

Answer 3

Feed your Pokemon poffins until it's smart maxes out.In a house in Pastoria city(i don't know witch one)talk to a women holding a parsol.She will give the green scarf.Works the same with other contest stats. Source: Experience This is Pokemon platinum's answer.Sorry for the inconvenience.

Comments

My answer works for platinum.I think it works for diamond too.

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Nope. It's a guy.

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Ok this is pokemon platinum's  answer