If you think you want to come back to an editable working copy of the grayscale version, save your file as a PSD now. Next, duplicate the image (Image > Duplicate) and flatten the layers (Layer > Flatten Image).
Go to Image > Mode > Indexed Color and adjust the settings as shown in my screen shot.
- Palette: Uniform
- Colors: 3
- Transparency: Off
- Dither: Diffusion
- Amount: 75% (adjust for best results)
- Preserve Exact Colors: Off
Play with the "Amount" setting for the best results. When you are happy with the black and white version, click OK.
Save the image as a TIFF, GIF or PNG file. Do not save as JPEG, because the dots will blur.
