Replace fuzzy single color in sRGB transparent PNG

coloreplace.sh

#!/usr/bin/env bash

FILE=$(realpath ${1});

OLD=$2;
NEW=$3;

# Default fuzz: 25%
FUZZ=${4:-25};

########################
# 1. Flatten & color-replace original into clone
# 2. Extract alpha channel from original image into clone
# 3. Remove original, composite flat + alpha into output.
########################
convert ${FILE} \
\( -clone 0 -alpha off -set colorspace sRGB -fuzz ${FUZZ}% -fill "${NEW}" -opaque "${OLD}" \) \
\( -clone 0 -alpha extract -set colorspace sRGB \) \
-delete 0 -compose copy_opacity -composite ${FILE}