Convert a colour in every frame of a GIF to another colour

gif-greenscreen-converter.sh

#!/bin/bash
input=$1
file=$2
rmax=$3
gmax=$4
bmax=$5
[ "$input" = "" ] || [ "$file" = "" ] || [ "$rmax" = "" ] || [ "$gmax" = "" ] || [ "$bmax" = "" ] && echo "Missing arguments"; exit 1
cp $input $file
inc=1
r=00
for((i=0; i<rmax; i++))
do
   g=00
   for((j=0; j<gmax; j++))
   do
      b=00
      for((k=0; k<bmax; k++))
      do
         color="#$r$g$b"
         echo "Filling color $color"
         convert $file -fill "#000000" -opaque $color $file
         b=$( printf '%02X' $((0x$b+inc)) )
      done
      g=$( printf '%02X' $((0x$g+inc)) )
   done
   r=$( printf '%02X' $((0x$r+inc)) )
done