Generate duotone image using ImageMagick

duotone.sh

#!/bin/sh

# A value between 0-1.0
strength=0.6
white="#e8a2c9"
black="#813722"

# https://unix.stackexchange.com/questions/40786/how-to-do-integer-float-calculations-in-bash-or-other-languages-frameworks/40897#40897
calc() { awk "BEGIN{print $*}"; }
strength_inv=$(calc 1-$strength)

magick input.jpg \
  -fx "$strength * (luminance * #e8a2c9 + (1-luminance) * #813722) + $strength_inv * u" \
  result.jpg