Find a color wheel online. Put in the fixed colors that will have to stay as they are...walls, flooring, ....now....what does that tell you will work? I think you have to go with a tone in the floor tile. You can choose a solid of the darkest tones in the floor tile(is that maybe like a walnut color or a pecan color?), or the lightest tones. There is no ash gray, or white in your floor tile and that is why the fireplace tiles will clash. https://www.hometalk.com/2243817/painting-a-brick-fireplace-with-chalk-paint?expand_all_questions=1     This is a great technique, looks lovely and is cheap. There is a recipe online how to take the paint you may have on hand and create "chalk paint' finish from it. I hope you find something you like.

Definately not that tile use a large format tile or paint it see if that does the trick first before spending any monet on tile With those vent holes the tile will be a bit of a PITA to install

Yes, you are right, the skinny grayish tile propped up next to the fire place clashes badly with the floor tile. In this case, I'd recommend painting the brick. If you absolutely must cover the brick with tile, find an extra floor tile and take it and the mantel piece to the local tile shop for help in picking a tile that is slightly lighter or darker but in the same color family as the floor tile. Bring some samples home, and be prepared to order a few days in advance.

You're on the right track!

The scale and finish of the new mantel is just right. The proposed fireplace tile looks to have a bit of your floor color in it; there doesn't seem to be much of a clash. Consider pulling a larger sampling of the tile to broaden the true spectrum of color as that may help with your decision-making!

Best of luck with your project!