Create a premium mobile game app icon for a kids coloring game. Show a charming fairy-tale castle with oversized rounded towers, heart-shaped windows, playful flags, and thick smooth black outlines in a coloring-book style. The castle fills approximately 95% of the canvas in an extreme close-up. A large glossy pink marker enters diagonally from the top-right, with its tip actively coloring the drawing. The artwork is split diagonally: the left side remains crisp black-and-white line art on clean white paper, while the right side transforms into a vibrant fantasy castle with bright pink roofs, golden flags, turquoise windows, rich gradients, glossy highlights, and soft toy-like shading. The magical transformation occurs directly beneath the marker tip with colorful sparkles, glowing fairy dust, and tiny floating stars. Use a pure white background with subtle paper texture and a soft drop shadow. Render in a polished, high-end Pixar-inspired 3D cartoon style with thick readable outlines, premium glossy materials, bright studio lighting, vivid candy colors, exceptional readability at small app icon sizes, and no text, borders, or unnecessary background elements.