This dialog allows you to change the properties of the selected Boundary Line.
This is the width of the line in millimeters.
The color of the line can be controlled by the setting for the course overprint color. This is the standard setting. It is controlled by the checkbox "Use same color as course overprint".
However, if you want the line color to deviate from the course overprint color, uncheck the checkbox, and use the color button to choose a different color.