To create a black-and-white image, you can decrease an image’s color depth to 2 colors. If your image is already black and white (perhaps it is a line drawing or a photograph to which you applied the Threshold command), decreasing the color depth to 2 colors can drastically reduce the file size.
When you decrease color depth to 2 colors, choosing from the variety of options is not an exact science. Try out several different combinations and see which produces your desired result.
To decrease the color depth to 2 colors:
Choose Image > Decrease Color Depth > 2 Colors (1-bit).
In the Palette component group box, choose the color channel to use for the final image: Grey (the lightness values), Red, Green, or Blue. Choosing Grey produces the best results in most cases. However, if the image consists mostly of a single color, selecting that color channel may produce the best results.
In the Reduction method group box, choose a color reduction method. See About Color Reduction Methods.
Note: With the Error Diffusion method, you must also select the Floyd-Steinberg, Burkes, or Stucki option, which are algorithms for the dithering pattern. Try them out to see what result you like best.
In the Palette weight group box, select an option:
Weighted A weighted palette produces less dithering and produces sharper edges.
Non-weighted A non-weighted palette produces more dithering and softer edges.
Note: Dithering places pixels of different colors or grey levels next to each other to simulate missing colors or greys.
Click OK.
Understanding Color and Color Models
How Monitor and Print Colors Differ
Making a Palette Color Transparent