。
The cold north wind turned the leaves brown.
Literal
Cold north-wind [subject-が] tree [of-の] leaves [object-を] brown [to-に] made.
茶色にする ('to make brown') — [color+に+する] is the standard way to express changing something to a color. The transitive する takes the direct object 木の葉 ('tree leaves'). 冷たい is the い-adjective for 'cold' (to the touch), distinct from 寒い ('cold' as ambient temperature felt by a person).