Guard cells that flank the stomata are kidney-shaped and are green in colour as they contain chlorophyll, while the epidermal cells are chlorophyll-free and contain Red pigment. Red light helps stomatal opening by the process of photosynthesis. So they are tiny openings found on aerial parts particularly leaves that appear green in colour. Interestingly, different coloured petals of flowers often have stomata that are sometimes non-functional. Fruits also can possess stomata. Abscisic acid is a plant hormone that influences the colour of stomata.