三澤秋 桜雨降る right? I wish the rain falling on you is not cold. I hope the rain falling on you is not cold. You choose wish or hope. In this case , the rain should be "not cold",shouldn't be "not so cold".

This is inflected form. 冷たい(adjective) ＋ ない(negative) ＝ 冷たくない 冷たくない ＋ ければ(conditional form) ＝冷たくなければ I am not Japanese language teacher. That means if I were wrong when it comes to technically. I hope it helps you in somewhat.

冷たくなければいい means if the rain isn't cold,that will be good. Or,I hope the rain falling on you is not so cold. Tsumetakunai >>>tsumetakunakereba. (a conditional form)