Set the color index range to be used for producing images with
PGGRAY or PGIMAG. If the range is not all within the range supported
by the device, a smaller range will be used. The number of
different colors available for images is ICIHI-ICILO+1.
Arguments
ICILO (input) : the lowest color index to use for images
ICIHI (input) : the highest color index to use for images