[Therion] Redefining PENA...PEND to draw lighter

[Michael Wasmund]

Hello,
Does someone have metapost code to redefine PENA...PEND to draw grey instead
of black?

Why I need this:

My survey consists of 80% scanned bitmaps and 20% drawn scraps.
The bitmaps are "grey", not black (can't make them better using image
processing).
 The drawn scraps then stand out and look ugly compared to the bitmaps.

I know how to redefine individual symbols (walls etc.) to be grey instead of
black,
but rather than redefining a large symbol set I'd rather redefine the PENs -
if possible. 

Regards,
Michael.