I know how to convert mp4 to webm with ffmpeg:
ffmpeg -y -i me939371029.mp4 -r 30 out3.webm
But I'd like to use webm transparency. That guide uses Blender, but Blender's a desktop tool that's not easily automated and only outputs PNGs that must be converted to video. I'd like a command line app that accepts video in, a color, and a video out. E.g.:
some-app -i video.mp4 -transparent ff0000 -o video.webm
I'd be surprised if something like this wasn't already in ffmpeg, but I can't seem to find it.