another important thing in a building is looking at existing buildings and what you like best. i for one love gothinc and roman style churches. so i will use that as an example.
a church is always cross shaped at the base, so if you build one 5 basic squares are a good way to start. then there's the outter gallery surrounding the squares, where on the roof of those the supprt pillars go, so the squares get a nice and believable height, making it more imposant.
making supportive elements in buildings, any buildings. makes it more realistic, and in that way nicer.
and although you mentioned variety is good, wich it is. too much can make a building look sloppy, don't be afraid to have a wall mostly the same colour, little touches that break the whole, like windows or small decorative forms can do the trick already. and try keeping it controlled, not entirely randomised.
also, the most important thing in building, circles. if you can make a nice circle you can make anything. working with shape a lot, like with statues or complicated forms, like.. let's say a giant painted glass window with only two colours can help a lot. what i used for this was making ships, and then with the keel and the under shape with it, helps a lot with curves.