For any signs or hanging signs, front text or back text, any lines, modify it with /data command, like: /data modify block ~ ~ ~ front_text.messages[0] set value '{"color":"#FF0000","text":""}'

They are non-editable when there exist one line of color-specific empty text string. is_waxed is 0b.

Tested on Linux 5.19.0, and ext4 filesystem. Minecraft is not modded.

Edit(UTC 2023/11/02 03:40): They are non-editable only when the text string IS empty! If it is not empty, they are still editable.