excellent mariem you are sooo close to the complete right answer
i'll explain in details
the brown cat will be Rr
the black cat will be rr
so using the punnett square the offsprings will be Rr,Rr,rr,rr
so the resulted off spring will be one black kitten,and one brown kitten and the other Rr shows a special case of SHARING DOMINANCE where nither of the genes is dominant over the other but they share the charcters
so the female brown and black spotted kitten took the gene of the black and the brown chracter from both parents (this is a special case) where the combination of DNA in some offspring may show some special cases .