Need help with a color correction over old highlights

Hi everyone, I was searching the boards, hoping to find the answer to my question ahead of time, but I didn`t have much luck.

My hair was highlighted approximately 4 months ago. I was totally dissatisfied with my results (after having this done for about 2 years.) My hair is naturally dark blonde/light brown....or even a "dirty dishwater blonde". I want to permanantly cover up these highlights and go back to my natural color. I had a color correction done once with a semi-permanant color (lasted about 8 weeks) in the past and I`d like to achieve the same results. I have been applying a level 2 color every 4-5 weeks. I am happy with the results first off, but I do not like it when the color washes out and fades. It just looks so unnatural.

So here is my question. Is there a way that I can put a permanant hair color over my hightlights....such as doing the roots first then combing the ends? Will I need to "fill" in my high lights first before doing so...and will that affect the color of my hair that isn`t highlighted? I have a color in mind that I would like to use, but I would rather wait for better advice. I have been coloring my hair on and off for about 7 years now, but never covered up highlights.

All opinions are welcome. Thanks for your time.

Serena