If you're committed to a concrete counter, consider picking up the color of your floor and cast the top to look like a large slab of stone.

if we're not committed to a concrete countertop, do you suggest something better?

granit : in very light gray :)))

There are some beautiful eco-counter materials which would look great with these colors. Squak Mountain Stone countertops is what we used in ours

You will probably want to stay with something within the earthtones, adding some green accents may seem like a good idea but make sure they will still look good if you decide the paint color isn't working. If I was crafting the vanity counter, I would build the top with a gray base and accent with browns, specks of black and blue/gray, with a soft tan accent dispersed throughout. It will match both wall and floor tile, and be able to blend with almost any paint color. I would use green accents in towels, toothbrush, soapdishes, etc. to pull wall colors down to vanity. I'm glad to see someone interested in using concrete in their home improvement. It is a wonderful material.