just iron them until the collar is super straight. Oh, and if your going to wear a pullover over the polo, button it all the way up so it will stay straight
It sounds like you might be referring to inside a man's shirt collar. The only way to keep a collar from discoloring permanently is to treat it each time you wash it. You can either use a...
A polo shirt is T-shaped shirt with a collar, typically a two- or three-button placket, and an optional pocket. Without collar, a shirt can not be called polo shirt.