。
Leaves turn brown in autumn.
Literal
Tree-[possessive-の]-leaves [topic-は] autumn [at-には] become brown.
茶色 ('brown,' literally 'tea colour') is one of many Japanese colour words formed by suffixing 色 ('colour') to a thing of that hue. Other compounds in this family: 灰色 ('grey, ash colour'), 水色 ('light blue, water colour'), 肌色 ('skin colour'), 桜色 ('cherry-blossom pink'). The ~になる pattern lets these nouns cleanly express change of state.