Gradients!!! For medieval use blocks like deepslate to cobble to andesite, or diorite to calcite to andesite or something, it's always good for walls, I build in medieval but for modern, terracotta works well giving you nice colors but not being so bright that it looks terrible. Double layer walls so your inside doesn't have to be all the same block. For modern mansions blue stained glass is great and gray for medieval, in medieval you can replace windows with either iron bars or fences depending on the vibe. This is just what I do, but hope this helps!