Color Filler for Hair
A: Color fillers are typically tinted protein enrichment formulas that are designed to add substance to hair that might be damaged and weakened by color or other chemical processing.
You can also find neutral protein fillers that have no color of their own, but which bond with hair that may have been damaged by prior services and give the hair additional internal structure with which the color applied may bond to ensure richer, deeper color that lasts longer.
The problem many people face with repetitive color services, especially those that are high-lift or two-process color services (where the hair is bleached then toned) is that the hair can lose some of its substance in the process of having the base color "dispersed".
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See also:
How to color hair
Questions about damaged and weakened hair
How to make hair color last longer
Will no ammonia hair color fade faster than hair dye with ammonia?