Compound adjectives are generally hyphenated when they precede the noun they modify. When they follow the noun, they are two words. Those compound adjectives are the following.
blue-gray/blue gray, past-due/past due, open-minded, narrow- hearted.
- She designed a blue-gray living room set. (when there is a noun after adjective, we use hyphen)
- She designed a living room set that was blue gray. (when there is no noun after adjective, we don’t use hyphen)
NOTE: Some compound adjectives we can only use one form even there is a noun or no noun after it.