Reference Anthony's notes: http://www.imagemagick.org/Usage/color/#replace
I tried:
convert logo: -fill black +opaque white tmp.png
to make everything that was not white to become black. I got the following error message:
convert logo: -fill black +opaque white tmp.png
convert: unable to open image `white': No such file or directory.
possible bug in +opaque in IM 6.4.1-3
Re: possible bug in +opaque in IM 6.4.1-3
We can reproduce the problem and have a patch in ImageMagick 6.4.1-4 Beta available sometime tomorrow.