Converting from color to true black and white in ImageMagick?

How do I convert from color to true black and white?

If a field has color, I want to convert it to black.

Use the following command:

convert <input> -negate -threshold 0 -negate <output>

Threshold Dithering Methods

Threshold Images

The simplest method of converting an image into black and white bitmap (to color) image is to use -threshold. This is actually a simple mathematical operator that just provides a cut-off value. Anything equal to or below that value becomes black, while anything larger becomes white.

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If you actually want to convert all non-pure white colors to black, then I recommend that you threshold the negated image instead of trying to work out the right threshold value to use (one less than IM's current 'MaxRGB'), a value that is dependant on your specific IM's compile time in Quality, or 'Q' setting.

convert logo.png -negate -threshold 0 -negate threshold_white.gif

Source Threshold Dithering Methods