Anti-yellow shampoo is formulated specifically to counteract yellow or brassy tones that often appear in blonde or lightened hair. These unsightly hues can develop when hair is bleached or when natural red and gold pigments are exposed after color removal. Here's the principle behind how anti-yellow shampoos work:

1. Color Neutralization: Anti-yellow shampoos contain violet or blue pigments that work opposite to yellow on the color wheel. Violet or blue cancels out yellow tones when they blend together due to the principles of complementary colors in color theory. By depositing these cool-toned pigments onto the hair, the shampoo helps to balance the unwanted warm tones.
2. Toning Properties: Some formulas also include ingredients like optical brighteners that reflect light in a way that makes hair appear brighter and less brassy. Others might have purple or blue dyes that adhere to the hair temporarily, providing a toning effect until the next wash.
3. Pigment Control: The shampoo may also contain mild cleansing agents that are less likely to strip the hair of its natural oils or deposited color, helping to maintain the tonal balance.
4. Deep Cleaning: Some anti-yellow shampoos cleanse the hair to remove buildup from styling products, minerals, and chlorine that can contribute to brassiness.

To use effectively, anti-yellow shampoos are typically applied after bleaching or highlighting and used regularly in place of regular shampoo, although not necessarily every wash to prevent over-toning. Users should follow the instructions provided with the product, considering the frequency of use based on their hair's needs and the intensity of the yellow tones they're trying to neutralize.