Red light rear facing.
It doesn’t need to be thousands of lumens either, that is likely to be more disorienting to drivers than normal brightness. This is why folks plow into emergency vehicles with bright flashing lights. Have you ever seen drivers that have replaced their tail lights with super bright LEDs? It’s actually super distracting.
Also, anyone showing up to a group ride with a super bright white light rear-facing light I will beat to a pulp with my skinny little cyclists arms.