I have med/dark brown hair with blonde highlights. I would like to colour my hair all over in a very dark brown at home. Would the blonde hightlighs go green or will they cover OK.

Dyeing bleached blonde hair back to brown can result in a green cast. I recommend doing a two step coloring process in order to prevent this from occurring. Try using a reddish color on your hair first, then apply your desired color. Keep in mind that a red-base color will counter-act green.