Final answer:
Option (A), To blend gray hair in hair coloring, the percentage of gray hair should match the percentage of Neutral Base used in the coloring formula for effective results.
Step-by-step explanation:
To effectively color and blend gray hair, the percentage of gray hair equals the percentage of Neutral Base (NB) used in the formula. This principle ensures that the color applied helps achieve a uniform appearance by compensating for the lack of natural pigment in gray hair.
Permanent hair coloring typically involves a process including an oxidant and coupler compounds to produce the final dye. However, semi-permanent color can be used to blend in a few gray hairs, but permanent color is needed as the amount of gray hair increases.