I need advice on re-touching new hair growth. I'll explain my situation 1st...
Approx. 2 months. Ago I dyed my hair with Clairol Professional Premium haircolor 3NN (which was supposed to be a rich, medium brown) mixed with a 10-volume developer. Well, my hair turned VERY dark brown, almost black on the ends. Needless to say, I did not want to further damage my hair, so I have lived with the results. It has faded some over time, but is still a shade or two too dark for my preference. Now I need to re-touch my roots ( have some grey hair). I spoke with a Clairol representative that advised me to use Clairol Professional Premium haircolor 4NN with a 20-volume developer, apply to roots only for the required development time (25-30 min.) and then pull the color through the remainder of my hair for 2-3 minutes.
I'm fearful with going up a level of color from a 3NN to a 4NN and increasing the volume of the developer from a 10 to 20, will result in having a lot lighter roots than the rest of my hair. The Clairol representative seemed confident this would work with desired results. I want my hair a rich, chocolaty medium brown. My natural color (re-growth) is medium, ashy brown.
Any advice would be appreciated!:)

Sorry to hear what is going on. I am in a High School where I am also going through my cosmetology classes during high school. I am in 11th grade and know all the basics and more to hair, nails, and skin. If I was you I would do what you were told. If you use a 4nn with a 20 volume developer this will not only do a retouch on your roots but you have a possibility of making your hair lighter since you don't like how dark it is but it won't be much lighter. To do this you have to do just your roots leave it on 25-40 min. Mix the extra color with color treated hair shampoo and after you rinse your hair bring the rest of the color through all of your hair with the mixture but you have to mix well then condition. If you are afraid to do this do the same process with your 3nn color and volume 10 developer.

If your roots are dark and want go a little lighter, you will need to use a higher strength volume peroxide developer with your selected hair color. A 10-volume peroxide developer will not lift hair color. It's formulated to deposit hair color without lightening the hair. However, a 20-volume peroxide will lift hair a couple shades lighter. Using a 20-volume developer with your desired hair color in equal parts may achieve results. But since you've used a Clairol Professional Premium color, you should use the 4NN hair color to touch up your roots. It may help lift your re-growth color 1-2 shades. Be sure to leave the color on the rest of your hair for 5 minutes to refresh your hair color.

Thanks Cosmogirl09 & PrettyLady for your advice!
I will try the 4NN with 20 volume this weekend!