green screen to transparent with ffmpeg ffmpeg does a very good job of getting rid of a green screen, with a command like this:

ffmpeg -f lavfi -i color=c=black:s=606x1080 
       -i input.mp4  
       -filter_complex 
            "[1:v]chromakey=0x70de77:0.1:0.2[ckout];
            [0:v][ckout]overlay[out]" 
       -map "[out]"  output.mp4


but it replaces the green screen with solid black, whereas I would like it replaced with a transparent background, alpha = 0.

I tried setting the color to #00000000 (8 zeroes indicating a zero alpha), but that still resulted in a black background.

Strangely, when I set the color to say red, it gives a red tint to many unfiltered areas.

So I would like a transparent background, but I'm also curious what the color option is doing.
Output by MP4 defaults to the H.264 codec for the video. H.264 does not support alpha.

You can output to .mov with a video codec -c:v of png or qtrle. There are others but they have restrictions on resolution and framerate.

You can't use the overlay filter as it doesn't output to a pixel format with alpha. So, just the chromakey filter, and no color source.