How do I fix the subtitles so its normal white? The subtitles is black on the Plex Web App, Plex Android app, Plex PS3 app and Plex PS4 app. But the subitiles are normal on iOS. This is very frustrating! I can’t see the subtitles because of this.
I did not have this problem before. It just suddenly was black one day…
I have the same issue but only when i lunch the Plex webplayer. Dont know why… Annoying.
I found out that MP4 movies with MOV_TEXT subtitles doesn’t work well with Plex clients other than Apple devices. Really annoying that Plex do not have fixed this, but it seems like MP4 files with MOV_TEXT dosent play well other than Apple devices in general other places too.
So after some digging and searching I fount out I could just change the encoder from MP4 to MKV without any conversion or change of the quality of the file with a very useful tool called MKVToolNix. MKVToolNix can also be used to add subtitles or sync them.
In my case, I had converted DRM protected M4V movies I bought from iTunes to Lossless MP4 (Used a program called “Tuneskit” if your interested), but when I used MKVToolNix to turn the file over to MKV, the subtitles did not get along to the MKV file. So to fix this I found a program called My MP4Box GUI (only for Windows) that could convert over the MOV_TEXT subtitles from the MP4 movie to a SRT subtitle file. Then I could melt the SRT subtitle file with the MP4 movie file with MKVToolNix 
Now are the subtitles on all my movies working correctly with Plex on every client.
So if you get black subtitles or the subtitles are acting really strange because you have MP4 movies where the subtitles is read as “MOV_TEXT” by Plex, I suggest to try out what I did.
MKVToolNix (Mac, Windos, Linux and more…): MKVToolNix Downloads – Matroska tools for Linux/Unix and Windows
My MP4Box GUI (Only Windows): http://www.videohelp.com/software/My-MP4Box-GUI (Don’t use this program anymore, has a annoying bug! Check my 9. July edit down below)
Edit 14. June:
Here is another new subtitle extractor that works with both Windows and Mac and that has a easier and more user friendly interface.
[New] Free Subtitle Extractor (By TunesKit):
The official site: http://www.tuneskit.com/freeware.html
Mirror links:
Mac: http://www.tuneskit.com/download/FreeSubtitleExtractor.dmg
Windows: http://www.tuneskit.com/download/FreeSubtitleExtractor.exe
Edit 9. July:
It seems like the “My MP4Box GUI” subtitle converter program have a very high chance of messing up the time on the last text line in the subtitle files. I therefore higly recommend TunesKit’s option over “My MP4Box GUI”!
In my case, I had converted DRM protected M4V movies I bought from iTunes to Lossless MP4 (Used a program called “Tuneskit” if your interested), but when I used MKVToolNix to turn the file over to MKV, the subtitles did not get along to the MKV file. So to fix this I found a program called My MP4Box GUI (only for Windows) that could convert over the MOV_TEXT subtitles from the MP4 movie to a SRT subtitle file. Then I could melt the SRT subtitle file with the MP4 movie file with MKVToolNix
Now are the subtitles on all my movies working correctly with Plex on every client.
Do you keep a srt file embedded in your mp4 file? or do you keep 2 formats? mkv and mp4? I add my srt files to mp4 with subler, but i keep seeing MOV_TEXT files in my mp4.
Every movie I rip from iTunes converts over from their own M4V format over to a lossless MP4 format. The MP4 movie file then has the subtitles embedded with it, but in the weird “MOV_TEXT” format. The subtitles is not burnt in tho.
I then take this MP4 movie file, use a program (example “Free Subtitle Extractor (By TunesKit)”), and extract and converts the annoying “MOV_TEXT” format over to the SRT format. This subtitle is now its own file. The original subtitle in the MP4 movie file is still there. But to keep this new subtitle file in the SRT format in the actually movie file, the movie has to end up being a MKV file.
When I add subtitles to the MP4 movie files with "MKVToolNix", the SRT subtitles stays in its normal format, but the movie file turns to MKV without any change in quality. It just changes its container. The subtitle file needs to be added besides the MP4 file tho, because "MKVToolNix" can not see the embedded subtitles inside the MP4 movie file, thats why I need to extract the subtitle as its own file as I wrote further up in this comment.
I think adding SRT sub files to MP4 movie files as you said is not the correct movie format to use if you dont want the "MOV_TEXT" subtitle format.