Separating coloured clothes from white clothes is a normal laundering practice. Doing this prevents any coloured clothe that may fade from fading into the white clothes. Some coloured clothes do fade and one cannot really tell which of the coloured clothes that one is about to wash would be the culprit. So it's best to separate the coloured clothes from the white ones.

Also, white clothes are most times washed with some stain removing chemicals like bleach that would be harmful to coloured clothes should they be washe'd together. Washing the white separately would prevent the coloured clothes from been destroyed by some of the stain removing chemicals used for laundering white clothes