The tile looks pinkish here, but it has not been sealed. Tiles look much better when wet. Thanks

The diamond is actually a blackish pewter color. Would it be better to just replace the diamond with the same tiles already laid? Is it too late? Not sure if it would tear wall up. I have small accents of this tile on the other areas. Thanks!

ask your tile setter how much trouble it is to remove it. If it's not too much trouble or cost, have them take care of it now. You can use those left overs if not too high of a relief for a wall plaque or hot pot trivet, set into a coordinating material. my friend used to say; It's your movie. :=)

You can always buy something later to rest against that splash, for example when entertaining and not in the middle of daily cooking. Several temporary ideas are: anything mirrored, ceramic, porcelain, copper. I actually cook on an icky flat top stove with white subway tile. I bought a 6 dollar framed mirror and it's propped up in the middle. Not huge but it helps take the plain look away. Especially pretty to look at when kitchen is cleaned up!

Ok, thanks. I dread asking him. I've had a difficult time deciding on tile, pattern lay, etc. :/

Be a good sport and be direct. Have coffee, tea and maybe a few decadent pastries from the local baker. He's just a person hired to help you like what you are purchasing. Part of him is in the job and he wants to be remembered in a good light. You may recommend him to someone you know. That is how people stay working. Just be courteous and appreciative.

Sorry I referred to the other tile as field. Field tile is the name of any tile thats the majority or background tile, so to speak. Are they tumbled marble? It's never too late, after all it's your kitchen and you have to be pleased with the outcome. Trust your gut feeling on this, if it's already not something you love, best to get rid of it. Blackish pewter, (looks navy blue in my monitor, arrg!!!!) If you decide to keep the diamond shape I would definitely do a dark grout between the dark tiles. Light grout would make it look like 4 individual tiles and busier. Another picture showing more of the kitchen would definitely help.

Don't feel bad about asking him, it's not his kitchen. I really wouldn't care if it was a bit of a pain or not, and I know it's not. Just tell him you need a different solution because this is not turning out the way you had envisioned it would. Ask him what he would recommend as well. This way he'll feel like part of the decision was his too and not as likely to get his feathers ruffled. Covering it, just to mask it is not a solution.

Awesome help! I really appreciate it. I'll take another picture in a bit.

I think changing those 4 tiles to the same as the rest will make it too boring. You say in your first post that it feels too plain, even though you were aiming for simple.
I would actually add MORE of those black/pewter tiles, to beef up the pattern and make more of a statement behind the cooktop. Print out a few copies of this photo, and play with a dark pencil, colouring in squares until you come up with a pattern you like. I think this could be quite lovely, still be simple, but will give you the oomph you need.
Should be relatively simple for him to pop out some of these tiles and replace them with the patterned ones, since the size is the same.

You commented that you used this tile on your back splash else where in your kitchen I would suggest following that same pattern behind your cook top.

Not sure what the rest of your backsplash looks like, but here are 3 SUPER fast renditions done in paint...pardon the messy lines, but you get the idea!

I think part of the issue is that the backsplash tiles look cool-toned while your countertop and cabinets appear warm toned. Maybe some tile that would tie the 2 together would be a better choice? Check your local tile shop and take photos with you. Also, I'm not sure if its just the angle of the photo, but the hood above the cooktop doesn't look level!