You can always check your mic and camera on the pre-screen before starting a meeting. I’m assuming you’re using the public instance of Jitsi at meet.jit.si, right? Jitsi is webrtc-based, so the webrtc troubleshooter tool at https://test.webrtc.org/ will pretty much give you the information you need to make sure you have the necessary resources. I’m not sure of what you mean by it sometimes being red, but Jitsi still working because in the case of your camera for instance, some resolutions could be red (meaning your camera is not capable of transmitting at that resolution), but so long as you have at least one resolution working (green), your camera will work on Jitsi.