。
The leaves are turning red.
Literal
Tree-[possessive-の]-leaves [subject-が] red-ly are-coming-to-colour.
色づく ('to take on colour') is a specific verb for foliage or fruit turning colour. Here it's compounded with 来る via the ~てくる auxiliary, marking a gradual change that's approaching the speaker's present perspective — 'coming to be red.' Compare with ~ていく, which would push the change away from the speaker.