Because my books are so scattered, and I have them on a couple of shelves here and a couple of shelves there, I really can't organise them very well. I do have my Wordsworths together (all 3 of them!), and my Penguin modern classics (with the grey covers) and then my Vintages (with the white covers), then my Jane Austens (which are all Penguin Read Reds). After that, it kind of degenerates and they're all a mess, although I do have one shelf for sci-fi, which is badly overflowing.
I like the idea of colours, but if I had the room I would probably end up arranging alphabetically in genre or something.