Thank you all for the tips and great advice. The "brass free" sounds quite interesting; I definitely have a horror of having brassy hair. I just want to go back to my natural dark brown, no lighter than my natural color. Would highlights take on an orange tone if I am trying to dye them brown? Maybe I'll try a little strand test of the Natural Instincts brass free and if I don't like that, I'll have my colorist dye it for me. Why does hair always have to be so complicated?!
I agree with the suggestion to have it done the first time. I just asked my stylist to match my roots with a single process color so I could let it grow out and not have to highlight anymore. It turned out perfect and I haven't had it done since then (a few years)...
But if you're worried about brassiness don't pick anything with "warm" or "golden" in the name. Look for a "cool" or "ash" shade. They do have a violet undertone, but it won't make your hair purple. All the purple does is counteract brassiness.