Not just a single automation, but RGB bulbs are my most useful automation. I have bulbs sprinkled throughout the house, and I use them for tons of things!
Solid colors:
- A red light outside my home office turns on when my webcam is on, so I don’t get visitors during a meeting.
- Motion-triggered, dim blue night-lights throughout the house.
- A light in the entryway activates at dusk, and I must appease it before going to bed:
- Orange means the coffee hasn’t been prepped.
- Yellow means the back door is open.
- Aqua means that some automation is armed that could cause the garage door to open by itself.
- Red means the security alarm is armed, but there’s a window open somewhere on the main floor.
Flashing light patterns for ‘events’:
- Pulsing blue light if it starts raining heavily and the windows are open.
- Flashing red and orange when the doorbell rings.
- 3D printer events are prefaced with three slow ‘aqua’ flashes followed by:
- Blue: Printer booted.
- Green: Print success.
- Red: Print failure.
…and a bunch of others.
I made an integration to handle all of these events, so I can actually quickly filter them in Home Assistant to be able to say that by “a bunch of others,” I mean I have 32 total things that the lights show. 
The integration shows each defined ‘event’ as a Switch entity. Each event has colors, optional auto-clearing, priority, and other attributes associated with it. The integration allows lights to ‘subscribe’ to these events, so I can easily select which lights get which events. Then, I just flip the switches on when they’re relevant and don’t have to think about it further.
I shared my integration here in case anyone is interested: Color Notify: Simplify Showing Multiple Alerts on One Smart Bulb - Design Colorful Event Alerts and Pair Them with Your Smart Lights