The opposite of "giving the green light" can be phrased in various ways. For instance, it could mean to deny, reject, refuse, or oppose. The phrase "red flag" is often used to connote that something or someone is not approved, so it can be considered an antonym. Other antonyms could be "protest," "forbid," "halt," or "veto," as they all imply disapproval or opposition. These terms capture the essence of withholding permission or approval. When one fails to give the green light, it often means that they have reservations or objections to an action or decision. Therefore, understanding antonyms for "give green light" is essential for developing a well-rounded vocabulary.