While I personally quite like the contrast between the gravel and the sand, as mentioned already, they will likely end up mixing either while you are planting the tank, during water changes/cleaning, and/or from fish moving them around. You could try adding some lighter colored stones/pebbles instead of the gravel, because then at least if they get moved aorund it's easier to put them back in place.
I love it overall though, and I think it will look great when you get the plants in and they mature.