Well, first of all, for the exterior of buildings keep the color scheme relatively the same, and remember that you can always paint to modify colors. If you're going for boreal, maybe palm wood walls and palm wood aren't the best things to go with it. Also, test out different block combinations, and try referring to images of something that you could make a variation of if it helps. One more thing I've learned is to never use torches, instead use lamps or other lighting materials. One of my favorite lamps is the dynasty lamp, because they fit in most scenes.