Collard greens are the leaves. Cauliflower is the flower head. Flowers are often a different color from the leaves, especially in species that rely on animals for pollination. Additionally, cauliflower is white, in part, because of a lack of chlorophyll. If collard greens had no chlorophyll in the leaves, then the plant would not be able to grow. If they can’t grow, then they aren’t likely to end up on a dinner plate or for sale in a grocery store.