Comment by __zayne__

I like to get the duration of the base video using ffprobe, then use -stream_loop -1 to loop the overlay video on top of it, while using -t with the duration before the output to ensure it will only be as long as the duration of the base video. Here's an example:

    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