Simple past tense and past participle of mellow.(verb)
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Use "mellowed" in a sentence
"“I daresay,” agreed Lady Armitage, more sharply, but her expression mellowed into thoughtfulness."
"In a certain tenderness of light and coloring, the poems would recall the mellowed masterpieces of the older literatures rather than those of the New England school, where conscience dwells almost rebukingly with beauty ...."
"He was then probably in the revolt against too much literature in literature, which every one is destined sooner or later to share; there was a certain roughness, very like crudeness, which he indulged before his thought and phrase mellowed to one music in his later work."