grow light coverage

dying hair that has two colors?
My hair is overall very blonde, like, light light ash blonde. My roots have grown out a little less than an inch, and they are a medium-dark brown. I think I want to go back to my dark brown hair (darker than the roots), but I am afraid that the roots and the blonde will turn out different colors?
I may get it done professionally, but when it comes to going dark i usually like to do it myself, if possible. Will this happen? or will the dark dye have even coverage?
well i am in beauty school and i don't know if i would risk doing this at home...we learned in color theory that blonde grabs color easily therefore you might have two different tones. i understand it can be expensive to get your hair done professionally but i suggest going to a beauty college. many times they need models and charge very little but do an exceptional job while using professional products..good luck =)