I turn the title of a book violet if I have some audio/software attached (based on the tags). There aren't many of them; it's mostly the case of e.g. foreign language textbooks with extra audio CD, so it is good to know straight away there is something more than just the book.
I also color the language of a book (depending on the language). I have four colours defined for my four main languages. I don't know whether that's useful for anyone but I am a language junkie

.
I'm new to Calibre so I still have to figure it out what columns and colours I need; I gonna share them when I know

. This thread is really useful. But I do get a little OCDish as well

.