Each Pen has a specific color and line weight. You assign pens to ARCHICAD elements in their own tool settings dialog box.
Note: Your current On-Screen View Options (Bold Cut Lines, True Line Weight) may affect the on-screen display of your lines.
To assign a pen to an element, open the pencolor pop-up (in the element settings dialog box or in the Info Box of a selected element) and choose the desired pen for the element or one of its components - such as the cut line pen assigned to the slab in the following image:
The available pens in these pop-ups are derived from the Pen Set currently applied for this project.
See Pen Sets.