Take a look at these before and after photos. They might help you decide. Some people painted the entire thing uniformly. Others whitewashed the rocks and used a grout color that maintained the separation among the rocks. Interesting.

Thank you - so helpful!

I think the rock is beautiful as it is, and would leave it just as it is. I would repaint the walls instead.

If the room is now gray, over the color shown, it could be that the gray is not flattering the fireplace.

Put a shmear over it.