In security red generally means encryption/protection is active.

And then there is this: https://en.wikipedia.org/wiki/Tally_light

> In the active (on air) mode, tally lights are typically red.

> Some cameras and video switchers are capable of additionally showing a preview tally signal (typically green) for when the camera is about to be switched to and become the main source of video signal. Once the switch happens, green changes to red.