Dye in plants and flowers. The coloured pigments gave them their colour. Rarity's eyes widened at the realization. "Ideaaa!" She exclaimed loudly and her horn glowed.

"I just need to change the pigment inside the plant!" Rarity smiled and concentrated her magic on the inside of the leafs. There it was, the green speck in each and every leaf. She willed the pigments to turn purple.