Coloring previously color treated hair

Here is what I know, though I am not an expert.

Color does not lighten color.  So, If your roots are black and your base color is dark brown and if you put more color on your hair, you will not get good results, more like a two tone color.  You need to treat the roots as 'virgin' hair and the rest of the hair as colored. 

Do you want to get rid of the highlights?  If so, then maybe you can use color on the roots only for the recommended time to blend with the light brown and then either 'pull through' the ends for the last minutes or so to cover the highlights  or you can tone the whole head after the roots are done.  The highlighted pieces will take color different from the base and the roots, so beware of this.  The problem I see is getting the correct color, since black to light brown is tough without bleaching first then toning. 

Maybe someone else has some advice.  Remember, always test strip before applying all over permanent color. 

Good luck

braelyn