Green [of-の] leaves [subject-が] little-by-little come-ている.
Adds 少しずつ ('little by little, gradually'), a common distributive expression: 少し ('a little') + ずつ ('at a time, each'). The ずつ suffix is productive — 一人ずつ (one person at a time), 一日ずつ (a day at a time).