Plex does offers a selection of colors to use, but missing the most important one: Grey. Grey is the default sub color on pretty much all 4K HDR discs. Reason being that with HDR the brighter colors glow.

SRT Colors

tom80H Closed April 14, 2021, 11:05pm 4

Early 2021 clean-up: duplicate(s)

Topic		Replies	Views
[Feature Request] 2 Subtitle options (HDR and SDR) Feature Suggestions subtitles	17	571	April 15, 2025
Request - Having a choice for subtitles color. Feature Suggestions	2	109	June 10, 2021
Subtitles looking too bright in HDR streaming Feature Suggestions	33	4540	September 1, 2021
Add a light gray subtitles option for LG webOS Feature Suggestions lg-webos , subtitles	4	79	July 30, 2023
How do I change subtitle color or make them more dim? HDR subtitles are supernova level bright Smart TVs lg-webos	4	132	July 26, 2022