In my previous experiments with making colored ice balls, I found that the color from food coloring in water didn't evenly distribute, even though it made pretty patterns.
But when I visited the new MKT Bar at the Four Seasons in San Francisco, I had a drink with an ice ball with well-distributed color. It was made of cranberry juice.
Surprised, I froze some of my own ice balls by filling the molds with juice - I tried grape juice, cranberry juice, and Vitamin Water.
All had nice and even color distribution. So this is a good trick for adding additional flavor to cocktails that slowly infuses into the drink over time.