Synchronised lyrics include timestamps, one before each line.
If you only add "[00:00.00]" at the very beginning of the lyrics, they will be considered as synchronised lyrics, thus displayed with the custom colour.
External lyrics files have the .txt extension, whereas synchronised lyrics files are stored with extension .lrc.
But the extension is not sufficient: an .lrc file without any timestamp inside won't trigger the use of the custom colour.
If you want to convert all your .txt files, it will require some programming work to produce a batch script that:
- scans you music folder tree(s) for .txt files;
- inserts "[00:00.00]" before the 1st line;
- renames the file to .lrc.
I don't know about lyrics stored within the audio files, since I always use external lyrics files.
Or try posting in the wishlist tab if you can't live without a custom colour for the non-synchronised lyrics.