Head to your local beauty supply store and pick out the best color filler for you. In most cases, you'll want to choose a tinted color filler to restore previously removed pigment or to provide a base pigment for your new color. If you're just looking to fill extremely porous hair with protein, you can choose the colorless variety.
Read over the instructions for your color filler product. Although using color filler is typically a fairly straightforward process, you want to make sure you're doing it right or this step will be for nothing and you'll have wasted your time and money.
Put on your latex gloves and apply the color filler to your hair from root to tip. Make sure the color filler is evenly distributed throughout the hair and that you get every last strand. Missing bits of hair can result in premature fading in those areas or a less-intense final color result. Either way -- it'll look like you goofed, so get it all.
Leave the color filler on your hair for the recommended amount of time. The filler will deposit the color and proteins into your hair shaft and ready your hair to accept -- and hold on to -- the hair color to come.
When the time is up, rinse the color filler (unless the product directions indicate otherwise) and towel dry your hair to remove any excess moisture. Comb your hair with a wide-toothed comb to remove any tangles. Your hair is now filled and is ready for the next step -- the permanent color.
If your ends are especially porous, apply the filler to these first. That will give those ends a little extra time to soak up the color and proteins.
Trying to color your hair from blonde to brown without using a color filler is the number one way to end up with green hair. Before you dye, you need to use color filler in either orange or red to inject the right underlying pigment into your hair before applying the brown hair color.