Swimming pools, of course, are always lovely and inviting, but if you've ever seen a dark bottom pool you'll know that they have a little something extra, a deep and mysterious and enticing allure that other pools can't touch. Who could have known that the color of the bottom of a pool could matter so much? But it does — one of those little details that make such a big difference. A pool with a floor and sides in black or navy or what have you is warmed more easily by the sun, but also…