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Leaves turn yellow in autumn.
Literal
Tree-[possessive-の]-leaves [topic-は] autumn [at-には] become yellow.
秋には adds the topic-contrast は to the temporal に — 'when it comes to autumn (specifically), X happens.' Subtle but real: the neutral 秋に gives plain timing, while 秋には sets autumn up as a contrasted case. 黄色になる is the standard 'become yellow' — 黄色 as a noun + になる for change of state.