Is this "one way" during daylight hours and night time hours when indoor lights are on?

These films do work very well during the day when it's brighter outside than inside. At night, just like a curtain or sheer, there would be a little more view to the interior since the lighting conditions would be reversed. We recommend ordering a few samples to test. I will say the light colors offer more privacy than the dark. We have samples on our lobby glass and you can indeed see the difference!