Why is deshake background green and how do I make it black?

FFmpeg version: 4.3.2-2021-02-27-full_build-www.gyan.dev, Windows

When I apply the deshake filter to a video, e.g.:

ffmpeg -i input.mp4 -vf "deshake=rx=64:ry=64:edge=blank" -c:v h264_qsv output.mp4

Even though I specify edge=blank, the background ends up being a dark green, e.g.:

Why is that happening? How do I make it be black?

The filter docs say:

edge

Specify how to generate pixels to fill blanks at the edge of the frame. Available values are:

‘blank, 0’ - Fill zeroes at blank locations

...

My interpretation of "fill zeroes" was black, but that doesn't seem to be the case.

I think the answer below regarding YUV 0's being green is on the right track, but deshake seems to be forcing yuv444p and I'm not sure why.

For example, an attempt to convert to RGB, using showinfo to print the formats along the way (line breaks for clarity):

ffmpeg -i input.mp4 
       -vf "showinfo=checksum=false,
            format=rgb24,
            showinfo=checksum=false,
            deshake=edge=blank,
            showinfo=checksum=false" 
       -f null -

Outputs (abbreviated):

[Parsed_showinfo_0 @ 0000010c55ab2f40] n:  22 pts:     44 pts_time:0.733333 pos:   348574 fmt:yuv420p sar:0/1 s:1280x720 i:P iskey:0 type:P
[Parsed_showinfo_0 @ 0000010c55ab2f40] color_range:unknown color_space:unknown color_primaries:unknown color_trc:unknown
[Parsed_showinfo_2 @ 0000010c5860d500] n:  22 pts:     44 pts_time:0.733333 pos:   348574 fmt:rgb24 sar:0/1 s:1280x720 i:P iskey:0 type:P
[Parsed_showinfo_2 @ 0000010c5860d500] color_range:tv color_space:unknown color_primaries:unknown color_trc:unknown
[Parsed_showinfo_4 @ 0000010c5861df80] n:  22 pts:     44 pts_time:0.733333 pos:   348574 fmt:yuv444p sar:0/1 s:1280x720 i:P iskey:0 type:P
[Parsed_showinfo_4 @ 0000010c5861df80] color_range:tv color_space:unknown color_primaries:unknown color_trc:unknown

In other words, it's doing this:

input.mp4
    ↓  [yuv420p]
format=rgb24
    ↓  [rgb24]
deshake
    ↓  [yuv444p]   <--- ?????????
encoder / output

I'm not sure why that's happening. Just to confirm it wasn't something to do with the output format propagating backwards, I also tested this and verified it has the same issues (GIF output uses rgb8):

ffmpeg -i input.mp4 -vf "format=rgb8,deshake=edge=blank" -c:v gif output.gif

Also, along those lines, adding format=rgb24 again after deshake yields similarly confusing results:

input.mp4
    ↓  [yuv420p]
format=rgb24
    ↓  [rgb24]
deshake
    ↓  [yuv444p]   <--- ?????????
format=rgb24
    ↓  [rgb24]
encoder / output

I experimented a bit with the + option to -pix_fmt but couldn't get that working either: It's documented as disabling automatic conversions in filters, but I guess format filters also count as "automatic conversion" because it prevents those all from functioning as well.

in YUV color model a value of 0 for U and V results in Green color

https://en.wikipedia.org/wiki/YUV

you could try a color format that has 0 as black

without having tested it myself (and without knowing your pixel format), try something like -vf "format=rgb24,deshake=rx=64:ry=64:edge=blank"