I’m having serious trouble posting here. Getting all sorts of errors when trying to upload pics and files. Anyway, I’m attaching a zip file with the building with changing lights I’ve just made. You can unpack the ‘Building’ folder to your content folder in your project and test it out. If you open your project and in the content window find the ‘Building’ folder and run the level included, press space bar on the keyboard and the lights on the building will change randomly every time. The key to this is to create a building model with separate material index for each window.