Both the bright color and the sweet flavor are found only in winter (cool temperatures). Kale produces the same bitter/hot mustard oils as other members of the family (wasabi, radishes, etc.) in warm weather (when insects are active) as a deterrent.
Kales are usually biennial, and in favorable climates (winters with only mildly freezing temperatures) they produce sugar in their sap to prevent it from freezing. Garlic and onions also use sugars as a natural antifreeze in their bulbs, which is why fried onions tase so sweet once their sharp-flavored essential oils are cooked off.
Planting flowering kales early enough in cold-winter regions will cause it to bolt in the summer. Late-summer sowing, and growth in a cold-frame, might allow you to produce your own in October/November... depends on how harsh your winters are. At a guess, roots might be grown, stored in a root cellar, then 'forced' like Belgian endive over the winter. Worth a try...