DURATION="$(ffprobe -i input.mp4 -show_entries format=duration -v quiet -of csv='p=0')"
ffmpeg -i input.mp4 -stream_loop -1 -i overlay.mp4 -filter_complex "
[1]colorchannelmixer=aa=0.1[2];[2][0]scale2ref[2][1];[1]
[2]overlay" -t $DURATION output.mp4