Small and medium-sized fresh air room A Dai Xiao Ai has black hair, while men are bald. Why is Nini the only one with brown hair?

In the cartoon, A Dai has black hair, and the big eyes of a young girl Xiaoai are black and straight, which is the standard for a big lady. As a little girl, Xiao Kui is also a child of Guang Zhi and Meiya, and she has yellow hair instead of black hair or dark brown hair, which is very cute. As a little girl, Nini wears two pigtails and her yellow hair is even cuter. Maybe Master Yoshito Usui thinks so, haha.