Mix the filler shade with a 20-volume developer in a color bowl, following the product directions. Yes, 10-volume developer is usually used when you're depositing a darker color, but in this case you want the 20-volume developer to open your hair's outer layer and really, really get that red or orange in there.
How you apply the filler to your hair is just as important as doing it in the first place. You want to start applying the filler 1 inch away from your scalp first. If you have short, healthy hair you can apply the filler hair color all the way to the ends in this step. If, however, you have long hair and/or your ends are a little bit dry you want to stop applying the color, at this stage, 1 inch from the ends.
Once you have applied the filler shade to the mid-shaft of all the hair, immediately go back and apply the same hair color to all the roots. You wait to apply color to the roots because the heat of your scalp will cause the hair color to develop quicker than the hair in the middle. This will result in flaming red or orange roots and a not-so-flaming mid-shaft.
Now that the roots have color applied to them, apply the filler hair color to the ends, if you left them out in the first round of coloring. The reason behind applying color to dry ends last is they are more porous and will take the filler color very quickly. Again, you need that filler to be evenly colored from root to tip for an even brown in the end.
Set a timer for 20 minutes. Ideally, the hair color on your mid-shaft will have been applied 10 to 15 minutes ago, and your roots approximately 10 minutes ago. If you feel it's going to take you much longer to get all the hair color on the hair, ask a friend to help you with half the hair to speed up the process.
After 20 minutes has elapsed rinse all the filler hair color out of your hair. Next, do not panic! Yes, your hair will be flaming red or orange right now, but we promise it get's better from here. Towel dry your hair to remove any excess moisture and run a long-toothed comb through your hair to remove any tangles.
Mix together your brown hair color now with a 10-volume developer, following the product directions. We're using 10 volume her because you don't want to open the hair shaft with 20-volume and risk loosing any of that red filler.
Now apply the brown hair color of your choosing in the exact same manner as you applied the filer: mid-shaft, roots and ends. Set a timer for 20 minutes and let that brown mingle with the red or orange.
After 20 minutes has elapsed rinse the brown hair color off thoroughly. Apply a conditioner for colored hair to to your tresses and let that sit on your hair for two minutes to seal the cuticle of your hair. Rinse and marvel at your fabulous new brunette hair color.
When purchasing your filler color your best bet is to visit a beauty supply store and tell the consultant there what your plans are. They'll help you choose the best filler shade and accompanying brunette hair color.
If the ends of your hair are very dry, keep a close eye on them while you have the timer set for the 20-minute processing time. If it looks like the filler hair color is matching the rest of your hair before the 20 minutes is up, rinse it out. The key is an even color, so let your eyes be your guide.
Don't use a semi or semi-permanent hair color for either the filling or coloring stage of this process. You need permanent color that's going to last, or this will not turn out as you'd hoped.