What do you think of this Tiffany's necklace? Can it really match with any casual/dressy outfit?

That necklace would match anything! I think it's really cute and your only real worry is for when it tarnishes because silver tarnishes quickly, but if it starts tarnishing just buy a silver tarnish remover cloth and it will look like nice again. Anyways that necklace is short and not very flashy so it could work for literally any type of outfit whatever color you wear. It's the type of necklace you could wear on an everyday basis if you wanted to. The fact that it has blue on it does not mean it won't match things that aren't blue; it's just like having a colored gemstone on a piece of jewelry - you can match it to your outfit if you want or you can just wear it with anything and it will still look nice/classy.