The toolbar for the Ink tool includes two other buttons: The undo and redo buttons allow you to adjust any marks made while you're making the annotation. Once the Ink tool is dismissed, undo and redo will no longer function for that annotation.
You can change the thickness, color, and opacity of the Ink line by tapping the Ink annotation button while it is active.
With the Zoom Ink tool, tap anywhere on the screen to start drawing in detail. Drag the handle in the bottom right corner to reposition the zoom box, if needed.
Editing an ink annotation
After the ink tool has been dismissed, you may edit the annotation to change the color, thickness and opacity of the annotation. To do so, tap the annotation, then tap "Style" in the upper left corner of the comment callout.