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The indication of an allergic reaction during a hair coloring service is primarily observed through redness or itching at the application site. This reaction occurs because the individual's skin can become sensitive to the chemicals found in hair coloring products, especially those containing ammonia, peroxide, or certain dyes. When a person is allergic to these ingredients, their immune system may respond inappropriately, leading to inflammation and symptoms such as redness, irritation, or itchiness.

In contrast, increased shine in hair generally indicates a positive outcome of a hair treatment, and dryness of the scalp can result from various factors such as product buildup or environmental conditions, rather than an allergic reaction. Unusual hair growth is not typically associated with allergic reactions and could be related to hormonal changes or other unrelated factors. Thus, the presence of redness or itching serves as a clear and direct signal that a client may be experiencing an allergic response to the hair color applied.