Green flowers are rare compared to flowers of other colors because one of the main purposes of most flowers is to differentiate the reproductive anatomy of the plant from the rest of the leaves. This helps attract pollinators like bees and wasps to help the plant develop seeds.  Most of the green flowers found in nurseries are cultivars of a plant that occurs with colored flowers in the wild. By deliberately breeding domestic plant mutations that wouldn't thrive in the wild, humans have developed green strains of many popular houseplants and flowers.  Night-blooming flowers are more likely to be green or lack color in their flowers. These flowers use nocturnal pollinators like bats that don't see color and wouldn't benefit from being attracted by colorful flowers. Instead, many of these flowers have a strong fragrance to attract pollinators.