You have several options when it comes to the interior finish of your swimming pool and spa. A quartz aggregate product called DiamondBrite manufactured by SGM is a very popular option. They offer the 17 colors shown below, and you can see how the color finish you choose plays a very big role in the color reflected in your pools water.
As you can see there is quite a range in colors and shades to choose from in just this one product. The darker colors like Onyx and Midnight Blue while beautiful will run you a bit more cost wise.
Another option is a pebble finish. Again this product comes in various colors, as well as sized aggregates. The aggregate gives a comfortable texture and a naturally beautiful look.