It really isnt an ideal situation, but a friend had to do something similar, so Ill try to describe what might help you. Find a color, or two, of tile that complements what you have. say say, maybe a medium gray and a white, of similar sheen to what you have, then have a good tile person cut into a mosaic, where the pieces are approx 2x2” or therabouts. It might look intentional that way.

Side note: it seems a bit strange that in your photograph, the cement strip looks even with your tile floors…. if that is true, Im not sure how to tile on top of it without creating a trip hazard. Maybe, in that case, just some artistic paint for that strip.

Dealt with similar issues in my house. Have a piece of wood milled to slightly overlap the tile edges and paint to match the tile.

A marble threshold?

I think real wood stained or gray washed similar tones to blend between the tow tiles might work.

I would do something completely different in size, but whose colors would blend. Maybe something like this :Porcelain mosaic

Thank you for all your ideas so far. The cement is actually lower than the tiles as the tile there was pulled out. Wood stained may be an idea. The pocelain mosaic is a great idea. I think I may order a sample.

(Just a little note here, that I'm so impressed by everyone's ideas!)