If you put a brown home hair colour on your hair you cannot know 100% what the result may be. Blonde highlights that are white and platinum in colour need to be coloured first with a warm shade to fill them with the necessary undercoat for a brown shade to be applied over the top. If this is not done then the hair may turn out greenish or very flat and monotone.
Highlights are expensive in the salon as it takes more time to do. Go for a consultation with your colourist to see how much they would charge you to get you back to your natural dark shade.