You have to max out a pokemons "coolness" with pokeblocks then go talk to the chaiman a the Pokemon fanclub in Slateport. It's the same with the other contest catagories and scarfs.
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im not so sure... but i think that when u go to the Pokemon fan club in slateport city and talk to one person, ur first Pokemon in ur list should be good in beauty contests for blue scarf, smart for green scarf, cute for pink scarf, cool for red scarf, and tough for yellow scarf. or at least that's kinda how it is in ruby and sapphire, except that they can only get one scarf.
To get a blue scarf in Pokemon sapphire you must take the Pokemon you want to hold the scarf in Slateport city and enter the Fan club.Your Pokemon must have maxed beauty,smartness e.t.c depending on what scarf you want to take.To maximize the beauty,coolness e.t.c you must feed it high level pokeblocks and you can make those from dry berries.Check the berry's description and you'll see which is dry.
You can't catch a poliwag in Pokemon Sapphire. You have to trade from Pokemon Fire Red or Pokemon Leaf Green.
you can not catch scyther in Pokemon ruby,emerald or sapphire . you have to trade them from Pokemon fire red or leaf green.
As far as I Know Chansey is not on the Pokemon Sapphire you can trade it from other versions like fire red or leaf green