Harmony is green, not just because she's a witch, but because she is extremely environmentally conscious. She eats only locally grown, organic foods and uses eco-friendly products for her lair. Even her flying broom is made from sustainably managed magical forests. She plans to make everyone green once they try her cursed organic apples.
Each mastery level adds another card to the cardpool for defeating Harmony. Each of the bronze, silver, and gold mastery levels unlocks a new card, while it takes all three diamond and onyx levels to unlock a single card.