I'm guessing you have additional tile left over. You should anyway for future repairs if needed.

My suggestion is to make up 3 "panels" of the tile on say a piece of plywood. Whatever thin set color was used to apply the tile to the spa should be the same. With the tiles being a bit transparent, the thin set color will show through.

Grout each with the different colors, and allow them to fully dry.

Place all 3 next to the spa in the daylight, and exam the color differences. This should help in your decision making. The coping will cast a slight shadow at the top of the tile so have someone hold the tile panels against the spa, under the coping, to duplicate the conditions.

Consider your plaster finish coordinating when making this decision also.