Formal:
* J'ai les cheveux châtain clair. (This is the most literal translation and is considered formal.)
Informal:
* J'ai les cheveux blonds foncés. (This is a more common and less formal way to say it.)
* J'ai des cheveux brun clair. (This emphasizes the "brown" aspect of the hair color.)
* J'ai les cheveux châtain. (This is a more general term for brown hair that could be interpreted as light or dark.)
Other options:
* J'ai des cheveux couleur miel. (This emphasizes the "honey" color of the hair.)
* J'ai des cheveux couleur noisette. (This emphasizes the "hazelnut" color of the hair.)
Ultimately, the best way to say it depends on the context and your personal preference.