We're putting in a new pool with OK flagstone for the coping and used canyon river 6 x 6 tiles. Here is a link. Our tiles have much less blue in them - more taupe and sand.

http://www.classicpooltile.com/6x6stone.html

we're going to use pebble sheen ocean surf. Here is a picture in process - everything is a huge mess because of all the rain we've had (GA)

White based plaster finishes don't compliment natural materials, such as your flagstone choice for the coping. Mind you, my opinion.

Plaster finishes in the gray scale look best with natural stones and tiles that are of the same color scheme. Grout choices should be also in the gray scale. with gray grouts. NOT WHITE!!!!

Glass tiles with blues, greens, grays (light and dark) look fantastic . Click on my Houzz page and see the photo is a gray Hydrazzo Hatteras Gray with Limestone coping and glass tile with aforementioned glass tile 1x1.