My professor is using questions from other colleges for an assignment. Is this plagiarism?
No. Unlike for students (who have to demonstrate their understanding) and for research (which has to demonstrate an improvement to the state of the art in the field) there is no expectation of originality in homework questions, and there is no ethical problem with having multiple classes work through the same problem set. There are even arguments to be made that it is a good thing for students to be judged on a uniform basis.

It would only be a problem if your prof represented these questions as his own work e.g. when highlighting his teaching contributions in his evaluation or in candidatures for a new position. As a student, you wouldn't be in a position to know about this.

There might also be a copyright issue if your professor did not ask for permission to use these questions. But there is no reason to assume that he didn't.