Maybe a medium brown, so that you get some contrast from each? Alternatively, a very gray shade...

Thank you

Great suggestions, Acm! To give you some inspiration, here are some photos that have your color scheme. Hope this helps!

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I like the mid-tone idea. Something like natural oak might be too light. The "cool tone" browns (more like fawn or mushroom than "orange"/yellow) would look very handsome. Choose the colour of the island and the floors TOGETHER...that way you will get the undertones to compliment each other.