Until we implement a way to color-fill existing shapes, please use this workaround:
1. Draw a filled shape as normal.
2. Tap Undo near the top-right corner.
Alternatively:
1. Draw a filled shape as normal.
2. Tap the Eraser twice, make sure Erase Entire Stroke is enabled, and choose the smallest eraser size in the contextual section.
3. Zoom in and carefully touch only the border of the shape with the eraser. This will be easier if you drew the shape with a large pen thickness.