PLT files and printing with HP

Archived from groups: comp.periphs.printers (More info?)

In CAD there is different kind of shadows and lighter and darker lines,
example old house outer lines have been printed light gray and
new pipe lines are made black.
When turned into PLT file and sent over e-mail, HP Designjet 600
prints the (from dos prompt) all lines same black, there is no different
from the walls and new pipes.
Any help here ?