Best not to wash it. Some colour requires clean and some unwashed hair. At least if your hair needs to be clean they will wash it for you!
It is conditioner that in particular is bad, as traces of it will effect the degree that the dye stays on the hair. Some shampoos contain conditioner too, so beware of that!
My hairdresser asks me to go to my appoinment with hair that has been dampened, not washed with shampoo. She states it is better for taking up the dye. Not sure how true this is though.