I would try something in a handmade glazed tile with a bit of colour and shine to it. Either that or just use the same counter top material in a thinner thickness for the back. Either will look great!

color might be the easiest and it would look good. maybe not what you were thinking but try some samples. I wouldnt contrast the grout too much tho.

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Maybe something slate colored. I like the look of dark gray next to cream. Also if you want to go a bit bolder, I wonder how something in the green family might work here. Like a sage. Green is a very tricky color to work with so be sure to really view samples in mult lighting. Also, it’s kind of hard to see your floors in the photos so tough to truly discern. But here are some samp pics anyway

Sorry app made me sign in again and pics didn’t post