For color indices 0-15, the color representation you set will become the new default for as long as you stay in PLT. This means the HArdcopy command will show the colors you have set (if the device is capable). To restore the original default colors use set the Red color value to NO and then reissue the cpd command. Thus to restore the default color representation of color index 2 on your default /xs device, use:
PLT> SCR 2 NO PLT> CPD /xs PLT> P
Note, for PostScript files, color index 0 is *anyways* transparent and thus the color of the paper. I.e., there is no way to change the background color (other than loading different color paper in the printer) for PostScript files. If you want to change the background color, use the /PNG device.
PLT> SCr White
will always force the background to be white. Note, SCr Black is not recommended as it would require white toner and black paper in order for it to work with PostScript devices. SCr Default restores the default behavior where PGPLOT picks the background color.
PLT> SCR 0 1. 1. 1. ! Set the background color to be white PLT> SCR 1 0 0 0 ! Plot color index 1 in black PLT> SCR 2 0 1 0 ! Plot color index 2 in green