mix.sh

#!/usr/bin/env bash
set -ueo pipefail
#set -x

mix_channel() {
  value1="$(printf '%03d' "0x$1")"
  value2="$(printf '%03d' "0x$2")"
  ratio="$3"
  result="$(bc <<< "scale=0; ($value1 * 100 * $ratio + $value2 * 100 * (1 - $ratio)) / 100")"
  if [[ "$result" -lt 0 ]]; then
    result=0
  elif [[ "$result" -gt 255 ]]; then
    result=255
  fi
  echo "$result"
}

mix() {
  hexinput1="$(tr '[:lower:]' '[:upper:]' <<< "$1")"
  hexinput2="$(tr '[:lower:]' '[:upper:]' <<< "$2")"
  ratio="${3-0.5}"

  a="$(cut -c-2 <<< "$hexinput1")"
  b="$(cut -c3-4 <<< "$hexinput1")"
  c="$(cut -c5-6 <<< "$hexinput1")"
  d="$(cut -c-2 <<< "$hexinput2")"
  e="$(cut -c3-4 <<< "$hexinput2")"
  f="$(cut -c5-6 <<< "$hexinput2")"

  r="$(mix_channel "$a" "$d" "$ratio")"
  g="$(mix_channel "$b" "$e" "$ratio")"
  b="$(mix_channel "$c" "$f" "$ratio")"

  printf '%02x%02x%02x\n' "$r" "$g" "$b"
}

mix "$@"