We need info as to your style and pictures.

See, very blah.

Because the tiles could be original historic fabric, that's why. Anytime changes can be made reversibly they should be. It's about stewardship, not fashion.

Those ceramic tiles are about as "HISTORIC" as a ten year old pre teen. As the "steward" of the bland builder effect, go get some new tile, Want it to look good forever? Have a pro remove the mantel and surround.......install the tile or any solid surface you love and then put the mantel and surround back.

Cheap way?? Paint it, grout and all. What color? Nobody knows what else is in the room.

https://www.aestheticinteriors.net/blog/2019/10/23/fireplace-makeover-how-to-paint-tiles-including-the-grout

https://www.etsy.com/search?q=vinyl+tile+stickers+for+fireplace

Maybe try something like this?

Please Joe I really respect your expertise but those are not in any way historic .To the OP we still have no clue as to your style that is what we needed pictures for so the whole room please.

My style is anything that is unexpected, leaning toward transitional, eclectic, natural and colorful.