# Converting white on blue to black on beige:
convert source.png -level-colors white,\#200070 +level-colors black,\#FFF8E0 dest.png
# Converting white on blue to black on white:
convert source.png -level-colors white,\#200070 dest.png
# Converting black on white to white on blue:
convert source.png +level-colors white,\#200070 dest.png

http://www.imagemagick.org/Usage/color/#level-colors

This snippet shows how to replace the colours in a monochrome image, i.e. one which only contains two colours and the range of hues between them.

Replace colour range

replace, color

Bash