Legacy ImageMagick Discussions Archive

exec("convert original.jpg -monochrome new.jpg");

Or

exec("convert original.jpg -black-threshold 80% new.jpg");

I think the results will depend on the original image, you may want to do some other comands on it first to reduce the amount of colours etc.

For a solution that works for specific colors (or even a list of colors) use something like, the inverted color replacement solution given (roughly) in...
http://www.cit.gu.edu.au/~anthony/graph ... or/#opaque

Remove colors