创建一个互动填色应用，让用户可以在轮廓图上填色。

主要功能：
- 图片选择界面：展示提供图片的小缩略图，易于浏览且可点击选择。
- 填色界面：竖屏全屏展示所选图片。左上角包含：
  - 带确认对话框（‘确定要返回吗？’）的‘返回’按钮。
  - 用于将填色后的图片保存为PNG的‘保存’按钮。
 右上角包含：
  - 一个大红色圆圈显示当前选中的颜色（默认亮蓝色）。
  - 点击该圆圈打开全屏调色板，内含24种鲜艳颜色（以小圆圈排列）。
- 填色交互：检测图片区域边界以便填色，用户可点击或触摸区域用选定颜色进行填充。

最佳实践：
- 确保用户界面直观，按钮标识清晰。
- 优化图片处理，提高填色时的边界检测速度。
- 在颜色选择和填色之间提供流畅体验。该应用鼓励创意，为数字填色爱好者提供简单、吸引且令人满意的体验。

Create a simple interactive coloring application:
First screen:
- Display thumbnails of the provided images:
  1. https://img.freepik.com/free-vector/hand-drawn-zoo-animals-illustration_23-2151199715.jpg
  2. https://img.freepik.com/free-vector/hand-drawn-zoo-animals-illustration_23-2151199718.jpg
  3. https://img.freepik.com/premium-vector/lion-giraffe-hand-drawn_185029-508.jpg
  4. https://img.freepik.com/free-vector/hand-drawn-owl-outline-illustration_23-2149277379.jpg
  5. https://img.freepik.com/free-vector/hand-drawn-fox-outline-illustration_23-2149272718.jpg
  6. https://img.freepik.com/free-vector/hand-drawn-lion-outline-illustration_23-2149285355.jpg
  7. https://img.freepik.com/free-vector/hand-drawn-penguin-outline-illustration_23-2150582038.jpg
  8. https://img.freepik.com/free-vector/animal-doodle-outline-cute-giraffe_1308-81895.jpg

- Thumbnails should be small and easy to navigate.

After selecting an image:
- Display the chosen image full-screen vertically.
- Top-left corner:
  - "Back" button: returns to image selection after confirmation dialog.
  - "Save" button: downloads the colored image.
- Top-right corner:
  - Large red circle indicating the current selected color (default is bright blue).
  - On clicking the circle, display a fullscreen palette with 24 vibrant color options arranged as small circles. After selecting a color, close the palette.

Coloring interaction:
- Detect boundaries of image areas.
- On click/tap within boundaries, fill the area with the selected color.

Q: How should users confirm when they click the 'Back' button?
A: A simple 'Are you sure you want to go back?' prompt.

Q: What specific file format should the colored image be saved as?
A: PNG format.