The phrase "pall upon" can be defined as to become dreary, dull or lackluster. There are many synonyms for this phrase, such as "cast a shadow", "cloud over", "dull", "dim", "fade", "lose luster", "lose shine" and "subdue". These synonyms can be used in a variety of contexts, such as when describing the weather, an event or even a person's demeanor. Using synonyms for "pall upon" is a great way to add variety and depth to your writing, and can help convey a more specific tone or mood.