The term "brightened up" refers to a situation or object that has become more cheerful, colorful or lively. Antonyms for this phrase would be those describing a transformative process to dull, dreary or disheartening. For instance, words like dimmed, faded, dampened or muted would be antonyms to the term brightened up. Such terms signify situations or objects that have become less lively or less colorful, taking on a more subdued or somber quality. Other antonyms that can be used in conjunction with "brightened up" are words such as darkened, gloomed or tarnished, which represent a decline in mood, atmosphere or energy.