My wife and I have spent a lot of effort on exactly this topic over the last few months. We've spent countless hours online looking at samples and pics. We've visited several pools to see how the pebble/plaster coloring affects water color. Here is what we learned: There are many factors that impact water color, including pebble/plaster color, decking/coping color, reflection from house (house color), time of day, degree of cloudiness...and I'm beginning to think that moon phase and planetary alignment also have some non-trivial effects.
In the grand scheme of things, there are about 3 options for water color:
1. Light Blue (white and light color finishes)
2. Dark Blue & black @ night (black and dark color finishes)
3. Greenish-blue (brown/tan color finishes)
The subtle differences within these 3 categories are more dictated by weather and pool surroundings. So I wouldn't overly stress about it. Decide which of the 3 general water colors you want and then let nature make it beautiful. Even though a lot of people (us included!) stress over this issue, think about this: Have you ever really seen an "ugly" water color as a result of bad plaster color selection? As long as the water is clean, the final result almost always looks perfect.
If I'm wrong, please let me know....we haven't gotten to plaster stage yet, so I can still change my mind on our color selection!