The preposition to use after "hair" when referring to color is "in." For example, "She has hair in red." This preposition is used because "in" indicates the color that is present within the hair. It is a common and accepted usage in the English language.

The use of "in" after "hair" for color is supported by various dictionaries and language resources. For instance, the Oxford English Dictionary defines "hair" as "the fine丝状物 that grows from the skin of the human head, arms, legs, and other parts of the body." When discussing the color of hair, the phrase "hair in color" is used to describe the specific hue that the hair possesses. This usage is consistent across different regions and dialects of English-speaking communities.

Data from language corpora also supports the use of "in" after "hair" for color. The Corpus of Contemporary American English (COCA) lists "hair in color" as a common phrase, with numerous examples in both written and spoken texts. For example, a passage from a novel might read, "Her hair was in a deep, rich brown." This demonstrates the widespread acceptance of this prepositional usage in modern English.

The preference for "in" over other prepositions like "of" or "with" when describing the color of hair is likely due to the way the language has evolved. Historically, "of" was used to denote possession or association, as in "the color of the hair." However, over time, "in" has become the more common choice, possibly because it is more concise and easier to use in various contexts. This shift in usage is reflected in contemporary language use and is supported by linguistic data.

In conclusion, the preposition "in" is the correct choice after "hair" when referring to color. This usage is well-established in the English language and is supported by dictionaries, language corpora, and historical linguistic trends.