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
Request - Having a choice for subtitles color. Feature Suggestions	2	109	June 10, 2021
Subtitles looking too bright in HDR streaming Feature Suggestions	33	4497	September 1, 2021
[Feature Request] 2 Subtitle options (HDR and SDR) Feature Suggestions subtitles	17	564	April 15, 2025
Greatefull for gray subtitles. Thank you! Smart TVs	2	170	October 12, 2020
Subtitle colors not working? Smart TVs android-tv	15	915	May 31, 2021