When you export an image you may also choose to convert its colors to 'black and white' or 'gray scale' in the output document.

1.Select Configuration from the 'Options' menu (function key f8) and click the Images tab or
select Export from the 'File' menu (Ctrl E) –  see Image format file export. Select an image format such as BMP or PNG then click the Image options... button.
2.Tick the Force monochrome.
3.Choose a conversion method from the drop-down list of options:
oHalftone (Default option)
The image is treated as if it were composed of blocks of pixels (block size 4x4 pixels). The average luminosity of each block is matched to a shade of gray with similar luminosity. Gray shades are actually blocks of black dots on white – the number and pattern of the dots within each block determining its overall luminosity.
This assigns black pixels to colors with a luminosity below the threshold value, and white to the remainder. Enter a value in the range 1 to 254 (default value is 127, i.e. 50%).
The density in each quadrant of the image is computed and the image adjusted by interpolating between these values. The image is then converted to a 1-bit image using the specified threshold value.
oDither A method of error diffusion that takes into account the discrepancy between the original luminosity of a pixel and the monochrome value that the default threshold value of 50% would assign to it. The difference is propagated to adjacent pixels, resulting in a more randomized scatter of black and white pixels than basic thresholding.
Converts image to solid gray-scale shades rather than black-and-white dot patterns; select also 4-bit or 8-bit Smoothing for output to Monochrome images.