。
The leaves are turning red.
Literal
Tree-[possessive-の]-leaves [subject-が] red-ly are-coming-to-colour.
Identical pronunciation and meaning to the kana variant 色づく — 色付く is the fully-kanji writing. Older publications and stiffer registers tend to prefer fully-kanji 色付く; softer contemporary writing tends to use 色づく. Pure orthographic choice with no semantic shift. The verb pairs with ~てくる to capture a gradual change emerging into the speaker's awareness — 'coming to be coloured' rather than the abrupt change implied by ~てしまう.