Starch forms a complex with iodine which is very dark blue in colour. When iodine is added to chlorophyll, no dark blue coloration appears in the absence of starch. In the common experiment to show that starch is a product of photosynthesis, the green colour of chlorophyll does interfere with the dark blue colour of the starch-iodine complex.
Plants do give out carbon dioxide during the day due to respiration.
Unlike most animals, respiration is slower in plants. Plants require oxygen but do not have respiratory systems like those in animals. Instead, gases are exchanged at several places. Oxygen from the air dissolves in the moisture of the soil and then diffuse into the root hairs. As respiration occurs within the root cells, carbon dioxide is produced as a by-product.