But, it might be nice to have a +transparent somecolor option to turn all colors transparent except the one specified.
convert rmgcb.png +transparent green1 rmgcb_none_green_none.png
convert: unable to open image `green1': No such file or directory @ blob.c/OpenBlob/2439.
There is a current workaround as pointed out by Anthony at viewtopic.php?f=1&t=13602
convert rmgcb.png \
-channel A -transparent green1 -negate +channel \
rmgcb_none_green_none.png
