Thank you very much for the quick answerHiragana は is used for the particle "wa", so あなたのともだちのくるまはあおじゃないですか。 is correct.
Both can be correct. The meaning differs depending on the intonation of the sentence end. The former meaning is a falling intonation, but it's more likely "Your friend's car is blue, isn't it?". The latter is a rising intonation.