Don't choose back splash until you are done with new cabinet color and new counters.

I agree with both of the posters. Wait until everything is in to make the final decision, but a crackled medium grey would look nice. Or if you want some color look into a nice glass tile.

Did not see the flooring. You're right- the grey wont go with the floors (the countertop is probably not going to go great either...). In that case go for a blue-green glass tile.

Do you love these colors or are you doing this because everybody is doing white/gray kitchens? Your kitchen will be dated to a 2010 - 2020 kitchen.

first off, do not get the 4" piece up the back! secondly, wait until the cabs are painted and quartz is installed. things have a way of changing once your colors are actually in your room. I'd also make sure you have the proper lighting in the kitchen before choosing anything. you want the Natural White in the kitchen. the lower temperature bulbs will cast yellow over everything. (you could use some additional lighting in there. do you plan on putting in some pendants over the island? maybe adding a few more canned lights? Four is not enough for an area that size)

Unless there is an 8 week lead time on the tile you want, there's no rush. besides, that's going to be a lot of white if you pick white tile. please, just wait.