According to Styles 101, hair color filler is a substance, usually straight color pigment or a protein conditioner, that prepares damaged, dry or faded hair for hair color treatment. Using filler can result in more even color and prevent fading.
In addition to preparing damaged hair for coloring, filler is also ideal for hard-to-color hair and is often used in color correction applications. Colorists often use filler when they identify potential problems with the tone of the color or if they feel the color is likely to fade quickly after application. Filler is also commonly used before dying hair several shades darker.