Some people choose to color their own hair which is fine until they want to change the color to a lighter or darker color.This is when they run into trouble! That is when they come to a professional hairstylist and want us to straighten it out.
First of all some of the clients aren't always honest with their hairstylist,that they have colored their own hair.The hairstylist needs to know this because then she will know what she is dealing with.Color does not remove color,so therefore she either needs to strip it first or if the client wants hi-lites she will need to use bleach,which can damage the hair if not done properly.
Some of the colors have metallic dye in them which cannot be removed with bleach or a color remover which can be disastrous! I've seen peoples hair turn a flourescent green.
Again that is why your hairstylist needs to know what type of haircolor you have used at home.