In my experience, painting a fireplace is a one way trip. If you don't like the color, you are welcome to paint it again, but the damage has already been done. You could take the paint off, but I don't think you will be happy with the results. Any chemical wash will leave paint residue in the mortar that you won't be able to get at without destroying the mortar, and any scraping / sandblasting / etc methods will do as much damage to the face of the brick as the rest of your lovely home.
Sorry kiddo, but that's the way it is. If someone else knows of another way, I'm all ears.
Buying a house with painted brick is kind of like marrying someone with a few faults, thinking you can change them later.... (grins)