The pens, colors and materials used to depict MEP elements in your project are defined as attributes called MEP Systems. The MEP System attributes are defined or edited by you in Options > Attributes > MEP Systems (or use the MEP Systems shortcut from the MEP Toolbar).
These controls are analogous to those of other ARCHICAD attributes (such as Line Types). Create a new MEP System, or rename an existing one.
Define the MEP category for which you wish to use this MEP System: Ductwork, Pipework, and/or Cable Tray. (Check the boxes that apply.)
When you assign an MEP System to an MEP tool (using MEP Custom Settings panel in the Tool Settings dialog box), your available choices will depend on the MEP System settings here. For example, when assigning an MEP System to a Ductwork-type tool, only those MEP Systems will be available in the pop-up list which you have defined as “Use with: Ductwork”.