Is the park open to people that are not going to the concert and also the visitor center while the concert is going well.

Yes they are

Kirk, Red Rocks is open even when concerts aren't going on. Visit then, we were in awe!

During a concert they check your tickets before the visitor center, so I do not believe you would be able to get in while a show is going on.

Visitor center is open per their schedule and one can visit the center without going to concert