The situation is : I have many images of documents from scanning. I want to keep the document\'s main content - which is printed in color black (a small range of colors arou
To match all colors except black you can use +opaque "#000000".
+opaque "#000000"
In order to include a little range around #000000 you can try different percentages with the fuzz operator:
fuzz
convert input.png -fill white -fuzz 10% +opaque "#000000" result.png
Tested with ImageMagick 6.6.0-1 on Windows