When I was looking for a simple way to automatically dim the lights of my home theater when Plex starts playing, I found that none of the solutions really live up to my expectations.
As I already had a Homebridge setup up and running, I decided to write a plugin.
Homebridge-Plex adds your Plex Account to your Apple Homekit as a precense sensor. From within your Home app (or any other Homekit app) you can then assign automations, for example lighting scenes to trigger when Plex starts or stops playing.
You filter active sessions by user, player or both. This means you can for example have one presence sensor that is only triggered by your living room AppleTV but not your iPhone (you don't want the lights to change at home if you're watching something on your phone on the go). You could even have different light scenes for each user for the same player.
Head over to the GitHub-Repository for a quick start guide and feel free to open an issue there if you stumble upon a bug or feel like something is missing.
Oh and this plugin requires a homebridge installation. If you're not familiar, that's a small software effectively acting as a Homekit bridge, allowing you to add a variety of devices and software systems that don't natively support Homekit. You just need a computer that's always on (like your Plex Server) or a Raspberry Pi.