If you tap off the power and ground going to the reverse lights, then the camera will only work in reverse.
It is safe but the Avic will let you watch the backup camera while moving forward to keep an eye on a trailer or something. So if you want to be able to do that you would need it tapped off the battery or more specifically, something that has power when the key is on and only when the key is on.
I don't know colors, I took apart the back left brake light module and tapped off the wires in there (you still need the power to run to the Avic to know when the reverse lights are on to show you the backup camera automatically).