Head to your local beauty supply store and pick up two hair colors. First, choose the final brown hair color you want for your brand-new shade. This should be a permanent hair color. The second color you pick up should be a semi-permanent color and will depend on your first hair color choice. This second hair color is your filler color (so your hair doesn't turn green in the dye-back process). If you chose a light- or medium-brown color, then pick up an orange-y copper hair color. If you're final color is a deep, dark brown, choose a red shade. Also, pick up a bottle of 10-volume developer and some latex gloves.
Your first step is to fill those highlights with the orange or red color. If you don't put back in the underlying pigment that you took out during the bleaching process, you'll end up with that greenish-brown color you hear horror stories about.
Mix your filler color with the 10-volume developer, according to the product's directions. Blend the two in a color bowl, removing any lumps in the magic potion.
Apply this filler color, from root to tip, all throughout your hair. Make sure to really work the color into the hair, especially on all your highlights. Let the color fill in those light streaks for 20 minutes, then rinse thoroughly.
Mix your permanent hair color with the 10-volume developer in the cleaned color bowl. Use the cleaned brush to mix the color like you did with the first one. Now, go ahead and apply this permanent color from root to tip. Work it in, giving your hair a little massage in order to get that permanent color right into every nook and cranny of your hair.
Process this color for 30 minutes. Once the time is up, rinse thoroughly until the water runs clear. Slather on a conditioner for color-treated hair and rinse after one minute. Now marvel at your new, rich brunette hair color. You did it!
The filler color should be the same level or one lighter as your permanent color. So if you choose a level-five brown color as your final color then, your filler color should be a level five or six.
Don't forget to clean your bowl, brush and gloves in between the two colors.
If your ends are trashed and dry, you may want to leave them out of the color application process for the first 10 minutes. Dry hair sucks in color faster, and you may end up with ends that are darker than the rest of your hair. If you can spare the length, save yourself the hassle and just have those nasty ends cut off before you color. Overly porous hair tends to let color fade out faster, so even the score with a good haircut.