Rainbow chard is not a specific variety of chards but a blend of different varieties planted and harvested together (or sometimes commercially blended after harvesting.) It is a combination of green chard, white seemed chard, yellow and red. Cooking rainbow chard is no different than cooking a single color but note that red-stemmed Swiss chard does discolor the onions and garlic as it cooks and rainbow chard and green chard do not turn out as reddish.