Portrait of a metamorphosis

Amy had been working on her latest painting, but she knew that something was missing. It was her friend, Seth. After much persuasion, she managed to convince him to sit for her once. However she realized that she was running low on art supplies. The shop she usually visited was closed, leaving her in a bit of a predicament. Determined to find what she needed, she stumbled upon a peculiar shop that seemed to have appeared out of nowhere.
The shopkeeper, Luna, offered her a brush as a gift. Amy thanked her and left the shop, her heart filled with gratitude. The following day, Amy set up her easel and began to paint Seth. Everything seemed normal at first, but as she worked, she made a small mistake, accidentally smudging a bit of paint onto the canvas. To her surprise, the stain began to spread, not only the canvas, but on Seth himself.  She realized that the modifications she made to the painting were also appearing on Seth.
At first, the changes were subtle. But as the days wore on, the changes became more pronounced, and Seth began to transform into the feminine boy of Amy's dreams.