Please, make sure that you choose monochrome.ctb plot style in Plot dialog, not others. Color Plot Mode option should be selected:
Also, be sure that all colors in the drawing use the color name (yellow) or the color index (2). nanoCAD cannot plot in monochrome colors in RGB format (244, 237, 117).
If you are working in nanoCAD 5, please note that this version does not print in monochrome color rasters and objects created on a layout.