Well, there are indeed zones that are meant to be dark, and others that simply have an issue with being lighted correctly since the last update.
To try to solve this last case, edit the shortcut that launches the game, that is go to start-all programs-planeshift-planeshift select it and right click on it->Properties.
Search for (...) psclient.exe\" and change it for psclient.exe\" -fullbright
Please notice the blank space between \" and -
If that doesn\'t solve the issue:
Mostly all graphic cards drivers allow you to tweak a setting called \"Gamma\".
Right click on your desktop, select properties, configuration, advanced options and there navigate to the tab of your card, and search for the gamma setting, raise it a bit. There are some manufacturers that allow to save \"profiles\" so you\'d be able to load them using a shortcut on the taskbar, rightclicking on it and selecting it, (for instance called) planeshift, before launching the game; of course, when you exit the game you can restore the settings back to normal.