。
Leaves turn red and yellow in autumn.
Literal
Tree-[possessive-の]-leaves [topic-は] autumn [at-に] turn-autumn-colour.
紅葉する as a general statement of seasonal fact. に marks the time. The bare 秋に ('in autumn') is a neutral generalisation; the contrastive 秋には, by contrast, would put extra focus on autumn as a specific case worth distinguishing from other seasons.