Allow users to change the line color by clicking the color in the palette (the same as it does now for fill)
You can set the outline color of a selected object by picking from the color palette in the same way as you can set the fill color:
Fill color is applied with a left mouse click on the color in the palette
Outline color is applied with a right mouse click on the color in the palette (on Windows)
On macOS the secondary click or control-click will bring up a short context menu with the option to "set outline color".