This seems like a very specific question so i hope there is someone who can help me out.
Within FFMPEG i am trying to add (channel pack) 3 different grayscale videos in RGB within a webm or mp4 video. I got it to work to a degree via Merge planes.
This is what i tried:

ffmpeg -y -i video1.webm -i video2.webm -i video3.webm -filter_complex "[0:v]format=gray[r]; [1:v]format=gray[g]; [2:v]format=gray[b]; [g][b][r]mergeplanes=0x001020:format=gbrp" -pix_fmt yuv420p -t 00:00:03 output.webm

This is the result:

As you see the 9 is visible in the red and blue channel and so are each of the other channels

I have tried just extracting a single channel in a video, change the compression change the format change the color space, make sure that its all linear but each seem to have the limitation of some smearing/artifacts between the other channels.

Is it at all possible to add different grayscale streams in the color channels without any artifacts and if so in which direction should i look?

Any help would be greatly appreciated!