秋になると日はだんだん短くなり夜はさむくなってきました is not the correct translation for the “when” clause in “In the fall, when the days grew shorter and the nights colder”.
初霜が木の葉を変えていく is unnatural. We usually reverse the subject and the object in this context. I know this order of words sounds unnatural in English, but the natural way of thinking/describing in Japanese is “she watched the leaves colored in bright yellow and orange and red by the first frost”.
みていました is also unnatural, and we usually spell the verb in 漢字, not 平仮名. Instead of 見ている, I prefer 眺める because the connotation of 眺める is to enjoy the beauty of nature.
見ていました doesn’t mean “watched” (past tense), but “was watching” (past progressive tense). I interpreted from the English sentence that she used to enjoy watching leaves in every fall. However, she doesn’t do so anymore (maybe because she is too busy, or too sick??). That’s why I prefer 眺めた「ものだ」over 眺め「た」.