Yep, spot on. They are a simple overlay. The PS script uses higher intensity colours from the base tile, it does a pretty good job by default but as a result you tend to find, for example, roads are recreated as fully lit so manual touching up is required afterwards. It will give you a very good canvas to start with though. I’ll PM you a link once I get to my PC later.
Alpha isn’t used on night tiles, save them as DDS1, that’s all that’s required. Alpha IS used on terrain tiles to define water and is what gives you the reflective effect.