When resizing an image the colors sometimes collapse to one color only. This happens with convert but much more in PerlMagick and even more when using PerlMagick together with the CGI module. This problem was reported to ImageMagick directly and is said to be resolved in ImageMagick 6.9.0-4beta. See http://www.imagemagick.org/discourse-server/viewtopic.php?f=3&t=26859
Auto-assigned to maintainer kwm@FreeBSD.org
The port was updated to 6.9.0-4 yesterday, please try that. Please reopen if it still a problem.