The WebVTT specification, https://w3c.github.io/webvtt/, mentions that color (and any other formatting) of cue text can be set using the "::cue" CSS extension.
I tried the simple vtt file:
WEBVTT
::cue(#first_cue) { color: blue }
X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:0
first_cue
00:00:00.000 --> 00:00:10.000
Allo world!
without any success.
What is the simplest implementation that would allow coloring/formatting of cue text?