Drawing color shapes on a monochrome monitor

[paul@vault.WUstl.EDU (Terrance Paul McCartney)]

I've run into a strange problem with color.  My program draws things of a
non-rectangular shape to a yellow background.  I also need to erase those
things without disturbing the rest of the drawing (i.e. the thing to erase
is wrapped around something else which I don't want to erase).  

For the above reason, I did not use erase-rect since it would erase too much.
I simply draw over the shape with the background color (yellow).  This works
fine on a color monitor.  When I run it on a monochrome monitor, the parts
that I erased show up as black, even though the background (yellow) shows
up as white.

It seems pretty screwed up that if you draw something with the background
color, it shows up as a non-background color.  Similar effects can be seen
in grayscale modes.  Is this another bizarre feature of the mac, or is there
some fix to this?

- Paul