The antonyms for the word 'brighten up' are darkened, dimmed, dulled, gloomed, obscured, shadowed, clouded, and depressed. The opposite of 'brighten up' is to make something darker or duller. To darken or shadow something means to decrease its intensity, whereas, to obscure or cloud something is to make it murky or indistinct. Therefore, the antonyms of 'brighten up' pertain to a variety of conditions that give rise to negative emotions, such as depression or sadness. Keeping this in mind, one should always strive to use words that elicit positive emotions and avoid such antonyms that can dampen one's mood.