set display grey|greyscale|color
<black|white>
Sets the mode of the display.
By default, the mode is color, where shading and contouring
is done with a rainbow of colors. When using a monochrome display, these
colors may not map to greyscale in a pleasing way. When the mode is set to
greyscale, contours are displayed using a single grey level,
and shaded contours are done using a sequence of greyscales.
You may optionally set the hardware background color to black
or white. The default is black.
Issuing: set display grey white gives a result on the display
that is very similar to the output produced by gxps.