Bleached hair question PLEASE HELP!!!

I have a question I am going to the salon tomorrow to get my hair colored. The problem is I have bleached highlights and the rest of my hair is a dark blonde/ light brown naturally. I'm going for a medium cool/ashy blonde. I can't afford a double proccess so I'm going to go with a single proccess if that's possible. I explained it to the woman at the salon and she said she could do it...but my question is will the highlights turn lighter than the rest? Or can the colorist put it on my roots first and let them proccess for a bit before putting it on the bleached parts? Has anyone had this done and if so did the color look the same all over?? I'd really like to know before I waste alot of money....Please help!!!