The purple color comes from a combination of blue and red liquids.  Most common for the blue is blue Curaçao, Hypnotiq, and more rarely Maui Blue Hawaiian Schnapps or Blue Tequila. Curacao will give it a nice hint of bitterness and a subtle orange peel zest taste. When using Hypnotiq it won’t have that bittersweetness: it tastes a lot fruitier, the cocktail is often called Purple Haze Martini. The red most commonly comes from cranberry, pomegranate, grape juice, or grenadine syrup. Blackberry or blueberry juice will give straight up the desired purple color to the cocktail, but you will need a touch of bitterness to make up for the loss of blue Curaçao.