Change default value of system variable?

["K. Bowman" <k-bowman(at)null.tamu.edu>]

I prefer to plot with black lines on a white background, so in my
idl_startup file I include

!P.BACKGROUND = COLOR_24('white')    ;Set background color to white
!P.COLOR      = COLOR_24('black')    ;Set default draw color to black

(COLOR_24 is my own routine.)

This works fine until I change plot devices and then change back, viz.

flow>idl
IDL Version 5.3 (IRIX mipseb). (c) 1999, Research Systems, Inc.
IDL> PRINT, !P.BACKGROUND, !P.COLOR
    16777215           0
IDL> SET_PLOT, 'PS'
IDL> SET_PLOT, 'X'
IDL> PRINT, !P.BACKGROUND, !P.COLOR
           0         255

This means that I have to include code to set !P.BACKGROUND and
!P.COLOR in every time I switch graphics devices. Is there any way to
change the default value of a built-in system variable?

Thanks, Ken