What would provide an even base from which to work and to replace a missing primary color?

A filler is used to provide an even base from which to work, especially in scenarios where there are missing primary colors in the hair. Filler products are specifically designed to deposit color, achieving uniformity and ensuring that the subsequent color application adheres properly and results in the desired tone. This is particularly important when lightening hair or correcting previous color applications, as it helps to balance out shades and facilitate even color absorption later on.

A color additive, while useful in modifying the tone of a color, does not serve the same foundational purpose as a filler. A base coat typically functions as a protective layer or preparation for color applications rather than addressing color imbalances in the hair. A color corrector is designed to neutralize unwanted tones rather than to replace or create a uniform base. Thus, the role of a filler is distinctly focused on establishing a consistent foundation for further coloring processes.