Why is there a last a/ah after bonni? Why qubba bonni-ah, and not just bonni?
The "-a/ah" here is a sign of feminine form. So, as the qubba3a "hat" (the noun) is feminine, the bunniyya "brown" (the attributive adj. of qubba3a) should also be feminine. -- "brown" follows the gender of "hat". Hence, "brown" cannot be bunni.