Formal:
* Ich habe braunes Haar. (This is the most formal and grammatically correct way to say it.)
Informal:
* Ich hab' braune Haare. (This is more casual and commonly used in everyday speech.)
* Ich habe braune Haare. (This is slightly less formal than the first option but still grammatically correct.)
Important Note: In German, "Haar" (hair) is singular when referring to a single strand of hair, and "Haare" (hairs) is plural when referring to multiple strands or a whole head of hair.