The primary reason for using a color filler is to:

Using a color filler primarily serves to adjust porosity in hair. When hair is color-treated, especially when lightening or removing color, its porosity can become uneven. Some areas may absorb color more quickly, leading to an inconsistent overall tone. A color filler helps to equalize the porosity by coating the hair strands, allowing for a more uniform absorption of the color during the dyeing process. This is essential for achieving a consistent, vibrant color that applies evenly throughout the hair.

Enhancing hair strength, lightening existing color, and neutralizing unwanted tones are related processes in color treatments, but they do not specifically address the primary purpose of a filler. While fillers may provide some benefits to hair condition, their main function is to prepare the hair's surface for even color application by adjusting porosity.