The term grout is more commonly used with ceramic tile installations. What you have surrounding the stone at your fireplace is most certainly mortar. Where grout is used to fill joints between tile, mortar is used to join stone together. In addition to providing a solid bond between the stone it can also add to the design and interest of stone or brick.
In any case, you can change the color of the mortar a couple of ways. If the mortar is just a touch too dark and you want to lighten it up a wee bit then start with a 10% solution of muriatic acid. Apply the solution to the mortar joints only using a small bristle brush or similar device. A little of this mild acid on the stone or brick will generally not hurt so long as you don't allow it to remain for very long.
Allow the acid to remain on the mortar joints for at least five minutes or until it has stopped fizzing. Rinse the area completely and allow it to dry. You may need to wait a few days until the mortar has dried completely to determine if the process worked. If the mortar has lightened but the desired result has not been achieved, give the mortar another acid treatment and allow it to dry to check the results.
And remember, safety first. Wear rubber gloves and eye protection and make sure that there is plenty of ventilation in the area when working with acid.
Another way to change the color of the mortar is to remove about a 1/4" layer from the surface of the existing mortar using a hammer and chisel and apply a new layer of mortar in its place. Powdered pigment can be mixed in with the new mortar to create a color to suit your decorating needs. You should first make a few test samples of the mortar on a board to make sure that you have the desired color. And be sure to allow the mortar sample to completely dry since mortar will typically lighted as is dries.
The black metal that you are referring to is the front face of the prefabricated metal fireplace that was installed before the stone. We understand your unhappiness with the appearance of this metal trim, but recommend that you do nothing to alter it. This trim frequently contains intake vents or air exchange vents that may be negatively affected if surrounded by stone or tile. We believe that a safer way to deal with this problem would be by painting the trim with a heat resistant spray paint that is close in color to either your stone or the new mortar color.