Some of the beer are cold. (Beer is not the subject of the sentence. The verb must agree with the subject. "Some" is the subject of the sentence. "Some" same as more than one. Therefore, the correct verb to this sentence is "Some of the beer are cold.
As I said before, I wonder, the correct one is: Some of the beer is cold. It's because, the subject of the sentence is "Some of the beer" and not only "some".