palette_reclass.sh

#!/bin/bash
#
#  palette_reclass.sh
#  Lars Schylberg, 2020-01-03
#
#  example how to reclass a tif palette image with gdal_calc
#   and some glue with awk and sed
#

# land 59,89,95,255 and / or 60,89,96,255  ---> 1
# sea 109,178,200,255 / 138,193,212,255 and 161,205,220,255.  --> 0

INPUT_FILE=your_file.tif

CALC_RULE=$(gdalinfo ${INPUT_FILE}  | sed -e '1,/Color Table/d' | \
awk -F: '{if ($2==" 59,89,95,255") {printf "1*(A==%d)+", $1}
else if ($2==" 60,89,96,255") {printf "1*(A==%d)+", $1}
else if ($2==" 109,178,200,255") {printf "0*(A==%d)+", $1}
else if ($2==" 138,193,212,255") {printf "0*(A==%d)+", $1} 
else if ($2==" 161,205,220,255") {printf "0*(A==%d)+", $1};}' | \
sed '$ s/.$//')

# echo $CALC_RULE
# Result with your example:  1*(A==0)+0*(A==1)+0*(A==2)

gdal_calc.py -A ${INPUT_FILE} \
	--outfile=output.tif \
	--calc="${CALC_RULE}"

# create a vrt for the new processed file

gdal_translate -of VRT output.tif output.vrt

# make a new vrt color table, just the first two values are used

cat <<EOF > color_table.txt
    <ColorTable>
      <Entry c1="0" c2="0" c3="0" c4="255" />
      <Entry c1="255" c2="255" c3="255" c4="255" />
EOF

for i in {1..254}
do
   echo "      <Entry c1="255" c2="255" c3="255" c4="255" />" >> color_table.txt
done
echo "    </ColorTable>" >> color_table.txt

# insert the new color table in the vrt image 
# after the row that contains the word Gray

sed -i -e '/Gray/r color_table.txt' output.vrt

# change color interpretation from Gray to Palette

sed -i -e 's/Gray/Palette/' output.vrt

# make a real tif image again from the vrt

gdal_translate \
	-co COMPRESS=PACKBITS \
	-co INTERLEAVE=BAND \
	output.vrt \
	final_output.tif