Pen settings for different disciplines is a good way to do it (which I have often used) but requires a pretty high level of discipline in maintaining standards. The foolproof way is to overlay (in the layout) the engineering drawing, with all pens set to black, over a background floor plan, with all pens set to grey.
How do you get to a command to set all pens to gray or black? Is that in the printer commands?
I am trying to think of how to separate the plumbing layer from the plan. The plumbing layer is a separate layer so it could be copied and pasted on another plan.
This could depend some on if you are printing from Layout or Plotting direct from model space.
What version of ArchiCAD are you using?
With ArchiCAD 11 you can set any drawing you like as a background, or foreground Reference Drawing.( not called Ghost anymore).
Not only can you make the reference drawing any color you like with out fooling with the original pens, you can also control the level of opacity you want for the reference drawing as well.
Just like ghost stories, I don't think you can use reference drawings in a Layout(?) so you would have to print that out from model space. This is no problem because you can print it out as a .pdf and drop it into a layout looking like you want it.
How to create new pen sets is what you probably want to know.
It is so easy, you have probably already figured it out.
One other tip you may have no use for. If you save your drawings as a .dwg, then print to .pdf using the Adobe Acrobat, all of your layers will be editable in the .pdf. Very cool!
You can do some incredible things to your drawings with the Adobe Acrobat Professional.