I am finding that one of the secrets to getting interesting photos of crystals, with lots of detail and color, is to grow them thin.  The thicker the layer of crystals on the microscope slide the less light that gets through reducing the colors, and the more depth of field becomes a problem with getting the whole frame in focus.

One thing you can do to promote thin crystal growth is to place a coverslip over the solution on the slide.  This also impacts how the water evaporates which greatly slows the growth process.

The above images were of Epsom Salts grown under a microscope coverslip.