ffmpeg \
    -i over.mp4 \
    -itsoffset 5 -i base.mp4 \
    -filter_complex '
        [0:v]colorkey=0x1DFF13:0.5[ckout];
        [1:v][ckout]overlay=[outv];
        [1:a]adelay=5000|5000[adel];
        [0:a][adel]amix[outa]
    ' \
    -map '[outv]' -map '[outa]' out.mp4