Background color in raster widgets

[Sven N. Thommesen]

Here's a little puzzle I need help with:

I'd like to display my (heat)bugs on a white background rather than pitch
black. The black background looks pretty awful when I try to print or make
transparencies. (Actually, I'd like to be able to specify any legal X
color, but white will do for now.)

I have changed Raster.m in the places that seemed relevant (lines 32, 57,
84, 120). This gave me partial success:

In a zoomraster on which I display only (3 kinds of) bugs, the background
is now white! Unfortunately, on zoomrasters where I display both bugs and
some attribute value, the background is still black :-(

I have tried to read Value2dDisplay and its superclasses, but I just can't
seem to locate the spot where those squares get set to black.

Help, help!

--Sven