Head to your local beauty supply store and purchase a semi-permanent hair color that is either one or two shades darker than your highlights (to tone down, but still keep, your highlights), or a color one shade lighter than your base hair color (to color over highlights completely). You'll also need a 10-volume developer, a color bottle and latex gloves.
Mix the semi-permanent hair color with the 10-volume developer, according to the product directions, in the color bottle. Screw te cap on tight, place a finger over the nozzle opening and shake it until the mixture is fully blended.
Slip on the latex gloves and begin to apply the color to your dry hair, from root to tip. Use your free hand to massage the hair and really work that color into the hair. If you miss spots, those highlights won't tone or disappear, so be thorough.
Set a timer for 10 minutes if you're just toning the highlights, or 20 minutes if you're ditching them completely. When the time is up, rinse the hair thoroughly until the water runs clear and there are no traces of color left in your hair or on your scalp.
Put a little moisturizing conditioner in the hair and massage it into the hair and scalp. Rinse with cool water to seal the hair's cuticle and lock in the color, and you're done. Good job!
Choose a toner shade that is from the same tone as your base color. In other words, if your base color is warm (red- or gold-based) choose a warm toner. If your base color is cool (violet or ash), choose a cool-based toner.
If you're going from very light-blond highlights to dark brown hair all over, you may have to pre-pigment the hair with an orange-based (copper) color first or your highlighted hair might have a green tint to it. The general rule is if you're going to darken four levels or more, you'll need to pre-pigment the hair.