Hi @andyfmarsh,
There are a few different approaches covered in this comment. If you want full brightness then you can just configure a relay output on your Navigator (per the intro), then use the Relay Toggle Control section to set up a Cockpit Action (which you can assign to a joystick button).
Feel free to follow up if any parts of that aren’t clear 
Perhaps worth noting that you haven’t mentioned which flight controller board you’re using - the described toggling approach will likely only work with a Navigator. A similar approach may be more generally possible with RELAY_STATUS messages, but would require some adjustments to the logic.