What is a common consequence of not performing a patch test before hair color application?

Not performing a patch test before hair color application can lead to severe allergic reactions in individuals who are sensitive to certain chemicals found in hair dye. A patch test is a simple procedure where a small amount of the product is applied to the skin, usually behind the ear or on the inner arm, to check for any adverse reactions before proceeding with a full application. If a person is allergic to one of the dye's components, applying the product without this precaution could trigger an immune response, which can manifest as redness, itching, swelling, or even more serious reactions like difficulty breathing or anaphylactic shock. This highlights the importance of the patch test as a safety measure in the hair coloring process to protect clients from potentially life-threatening situations.

The other options do not directly relate to the immediate health risks associated with allergic reactions. For instance, increased hair growth is not caused by hair dye application, and brittleness may result from over-processing or improper care rather than from an allergic reaction. Additionally, while a change in hair color is the intended goal of dyeing, it does not encompass the potential dangers involved with neglecting to perform a patch test.