Merge video with subtitles

[snippsat]

If I understand correctly, you mean that with Subtitle Edit you can change the color of the subtitles, right?
I loaded .ass file in Subtitle Edit, but didn't find how to do it.

Yes,of course that some of the point of this program.
This took me a minute to figure out.i have only used Subtitle Edit to save as .ass before.
Just select all text(Ctrl+a) and can start with right click(Remove all formatting) then is clean text.
Then the same right click and choice Color,eg here i choose yellow and save the .vtt as test.ass

G:\1_youtube\arte_env\Scripts
λ mkvmerge -o geo_new3.mp4 "Geo Reportage [101139-013-A].mp4" "test.ass"
mkvmerge v68.0.0 ('The Curtain') 64-bit
'Geo Reportage [101139-013-A].mp4': Using the demultiplexer for the format 'QuickTime/MP4'.
'test.ass': Using the demultiplexer for the format 'SSA/ASS subtitles'.
'Geo Reportage [101139-013-A].mp4' track 0: Using the output module for the format 'AVC/H.264'.
'Geo Reportage [101139-013-A].mp4' track 1: Using the output module for the format 'AAC'.
'test.ass' track 0: Using the output module for the format 'SSA/ASS text subtitles'.
The file 'geo_new3.mp4' has been opened for writing.
'Geo Reportage [101139-013-A].mp4' track 0: Extracted the aspect ratio information from the MPEG-4 layer 10 (AVC) video data and set the display dimensions to 1280/720.
Progress: 100%
The cue entries (the index) are being written...
Multiplexing took 19 seconds.

Now is all text yellow.