Why can't you shampoo and dry your hair prior to coloring?

Even most professionals don't wash and shampoo your hair before they proceed with coloring. At first, I thought that they were just plain lazy. But I now know that even the instruction from over the counter hair coloring kits also asked you to do so. Wouldn't it be better if your hair is free of gel, mousse or hair spray before you color or highlighting it? Or does it not matter at all ? What is your best brand in your opinion ? I used multi-tone haircolor from Feria (L'Oreal), and the result wasn't bad. But I didn't get the ';multi-tone'; as advertised by L'Oreal.Why can't you shampoo and dry your hair prior to coloring?
For some reason ';dirty'; hair holds color better. I have actually washed that morning and colored that night and haven't seen any difference. The only multi tone color I've seen that works is the Herbal Essence in the greenish box. I used the ';copper'; color and it really did turn my hair into different strands of color. It looked awesome and very natural.Why can't you shampoo and dry your hair prior to coloring?
BEACUSE WHEN U WASH, ALL THE NATURAL OILS WASH OUT. AND ALL THAT BUILD-UP HELPS UR HAIR STAY PROTECTED FROM THE HARSH CHEMICALS OF HAIR DYE. WHEN U WASH AND THEN APPLY THE DYE TO UR HAIR UR ONLY CAUSING DAMAGE CUZ UR HAIR IS-WELL ITS LIKE BEING NAKED AND THE CHEMICALS JUST GO DIRECTLY ON2 THE HAIR AND JUST DRIES IT AND U GET SLIT ENDS AND UR HAIR STARTS FEELING ROUGH AND UNHEALTHY.
OK your right about that your hair should be free from mouse and gel. So here is what you are suppost to do starting the day before you color your hair: You wash your hair with shampoo and conditioner, but when you style you are not suppost to use any styling products or any hair products at all. If you want your hair straight just blow dry than use a straightening iron. OK so the day you are going to color you have hair that is free from products and today you want want to wash your hair, cause the oils from your hair help keep it from not getting so damaged and also helps the color to work better. I like L'oreal to but nothing in the box will give you two tone color even if they advertise it. For it is just a gimmick. So remember no products the day before and no washing the day of. Good Luck. Hope I helped.