I agree with you the pillars are unfortunate. The replacement tiles will stand out no matter how close they come to matching so you're better off making sure they are clearly different so the gray one is better. Putting busy mosaic tiles could look nice but will interrupt the flow between rooms which I'm guessing is your problem with the columns in the first place. You will need to create a dextrous solution to tying into the 2x4 wall on the left side but I think it can be done tastefully- maybe beveling it back once you get past he end of that wall.

Might I suggest a more contemporary picture frame mantel with mosaic stone tile between that and your firebox? Love the near craftsman window shapes flanking the fire wall . . . use the height on both sides, / shallow shelves on one side can visually frame a swing arm for your television and give real storage on the other side.

One option you could opt for is to get a marble mosaic flooring. It is very durable and easy to clean. Furthermore, you can get virtually any rug pattern, design and color palette of your choice incorporated. Bonus: Marble mosaics work great in high-traffic areas. Here are some of our intricately handcrafted mosaic rugs and medallions so you get an idea of how they look: You don't need to worry about the shape and size. That's the beauty of mosaic, it can be customized to fit any surface. See anything you like? Have a favorite photo or pattern you'd like to be converted to a mosaic masterpiece? We can help! If you have any queries feel free to contact us. For more designs visit our website: MEC Artworks. Regards, Nada Zain

I do not have any tiles in my current house but if you think about grout discolouring and looking bad.... then I think that might answer your question. My thought - the less grout the better.