I separate whites for the simple reason that I stick one of those whitener thingies in the machine - what we can do with white clothes in this house has to be seen to be believed!
Beyond that, everything gets washed according to type rather than colour. So I sort into towels, sheets, underwear etc.
I do have a friend who is so bonkers rigid about colour separation that she'll separate peach coloured clothes from pink ones. But then I suspect she has too much time to fill.