1.) Rare Candy can be used on a Pokémon the same turn its played beginning with the second player's first turn.
2.) Multi Energy provides Colorless Energy only when its attached to a Pokémon having another Special Energy card attached to it at the same time.
If you had a Multi Energy as the only Special Energy card attached to a Fire Pokémon, i.e. Magmortar (SW), it would provide Fire Energy. If you had a Rainbow Energy card and a Multi Energy card attached, it would deal 20 damage, i.e. Flame Blast.