It all depends on your skin tone. Being that you are a brunette, your skin tone is likely a medium. Stay in the darker shades. An auburn or mahogany would likely look great.
her hair is brunette
brown
Brunette.
Brunette
In order to change blond hair to brunette, you must get hair dye and dye your hair the color you want.
Her hair colour is ash blonde.
A hair colour could be brunette , red or even black .blonds are OK but not as good as the hair colour.
lilac
She's a brunette! (Brown haired)
Blue jeans, black heels, Black nails, brunette hair (if you plan on coloring it.)
John Cena's hair color is blond with brown hilights
If the colour of your natural hair doesn't compliment your skin tone, then maybe you should. But go for colours which blend in with your real hair colour. Otherwise, they do not look very good.