Quote:
Originally Posted by
Man Of Lint 
Go for colours that contrast one another but compliment the coat you are wearing.
Welcome to Styleforum.
Scarf: "Hi coat. You are very handsome."
Coat: "Thanks scarf. You are kind of cute, too."

Kidding aside, I agree with MoL. Also, similar but not matching hat and scarf colors with similar depth of hues can work as well, if that makes sense.