When zucchini become yellow, it can either be a normal part of the developing process or a warning that something is wrong with the plant. Initially, green zucchini fruits eventually turn yellow as they ripen. The fruits are still edible when they turn yellow. However, they may be a little sweeter and more sensitive than green zucchini. The zucchini fruit can overripen and become yellow if it is left on the plant for too long. The optimal time to harvest zucchini is while they are still young and fragile because overripe fruits are typically bigger, harder, and less tasty than younger fruits. Zucchini plants require pollination to produce fruits. If the flowers on the plant are not pollinated, they will wither and fall off, and the immature zucchini fruits will turn yellow and eventually rot. To encourage pollination, attract bees and other pollinators to your garden by planting flowers and avoiding the use of pesticides.