Why do I have roots??

Hello,

So about 7 years ago, I decided to get my hair done and dye my natural hair a darker color with a tint of red in it. Since then, my natural hair is a reddish brown (especially in the sun) but I have dark roots.

To combat the dark roots, i have been dying my hair dark the past few years. Recently, I decided I really like my faded red natural hair and got my roots done to match my hair.

After that happened, I wanted to dye my hair a nice medium to light brown so I wouldn't have to deal with roots and so that my hair wouldn't look as uneven. My hair stylist for the past few years has been dying my hair and when she first does it, its so dark it almost looks black. It obviously fades over a few months to a nice medium-dark brown but I really don't want it that dark.

I went to sallys because I wanted to dye my hair myself. I choose the brand age beautiful and a woman that worked at sallys told me I have to dye my roots as dark or darker than they are or the color won't take. She picked out 5N for me and sent me on my way. I just dyed my hair with the 5N and its so incredibly dark, just like all the other times. I really wanted a nice medium brown. I know it'll get lighter though.

So, my question is, if I have gotten my roots dyed light to match my hair before, then why can't I dye my hair all one color to maybe a 6-7N or a mix of the two? Why wouldn't it take to my roots if I have gotten that done before?

Also, I was wondering if it would look good to maybe next time mix a 6N with a light golden brown.

I posted a picture of my roots with the red, my dye after dying it dark, and my hair in its natural state