Seems very strange -- no way that's a focus wall, and I think the difference would look strange. (in contrast, having something coordinating but different behind stove is more common, but you don't really need that.) I think the contractor is looking for an excuse to sell you some more expensive tile, sorry.
If you want some whimsey, maybe stencil the back of your island with something picking up the yellow of your walls! :)

Do not go with 2 types of backsplashes in different areas. I'm not familiar with the color of the subway tile you mentioned, however a light blue, teal, or light green would be lovely laid in a running bond horizontal pattern, Or you could change it up with a vertical end to end, side by side application with using 2 or 3 rows of one inch glass tiles in a darker tone of color going vertically every 18 inches or so. But if going this way be sure to lay it out 1st so that the one inch tiles do not land where your light switches & electricals land.

I would not use a different tile in such a small area. As a prev poster mentioned, that is not a focus wall or anything. If you had a huge kitchen with different prep areas and wanted to set one off as different or something along those lines, it could be quite nice. But in this case I think it would just look mismatched, like an afterthought or you ran out. Go with your gut.

Personally, in viewing your kitchen, I would go with one tile throughout. I am sure it will be lovely. Our kitchen has 10' ceiling. Our vent duct is 30" plus the vent hood is 36Wx18"H - it was suggested that we consider using Tantrum Sliver Duo in that area only (from cooktop-behind venting), with stacked subway on the remaining wall between bottom of cabinet and countertop. I was told it will look "amazing" rather than all boring subway. Tantrum Sliver Duo is cracked linear glass and would be tone-on-tone with our subway color. I'm not sure about this....which is how I came upon your thread.

This is akitchen I did for a client with 2 very different backslashes on either side of the kitchen i really wanted to bring the red fromm the art to the other side so hence the red tile. they love it and so do I.