help with professional hilights

A few months ago I decided to finally take the leap of faith &get most of my naturally brown hair hilighted blonde. At first I couldn't stand to see myself in the mirror, then I sort of got used to it &now I just want my brown hair back! My roots are starting to show, but it doesn't look too terribly bad since the stylist left pieces of my natural color in my hair...it's sort of complicated to describe, but basically, the majority of my hair is blonde. I was wondering if there would be anyway to get a box color from the store &dye over it. I've heard from a few people that this would be difficult, considering the hilights would be a different tone of the color versus the natural brown pieces. I've also heard that it will make my hair look very spotty...several faded spots &other very dark spots? Any help and suggestions on this topic would be very apprecciated. (Also, professional coloring is mostly not an option as far as suggestions are concerned)

I had alot of blonde highlights in my dark brown hair to help cover greys. When I got too many greys to cover, I decided to stop highlighting. So, I had salt and pepper roots and growing out highlighted hair. I wanted my hair all one color. I went to Sallys and the girl there was a big help. She told me exactly what I needed to cover the mess I had! I used there ION brand and it covered everything beautifully.