Rainbow shard is not collards. You may be conflating the two. Collards are very related to kale. That same species, Brassica oleracea, gives us the cultivars cabbage, broccoli, cauliflower, kale, Brussels sprouts, collard greens, Savoy cabbage, kohlrabi, and gai lan.
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.