Take one sugar crystal and color it with food coloring. (It will grow a bit larger.) Next, put it in a jar with two cups of sugar. The ratio of the colored crystal and the rest of the sugar is approximately 1 part per million. Don't worry if the colored crystal gets lost in the sugar. If you roll it jar around enough the colored crystal will emerge because it is slightly heavier than the rest of the crystals.
source: Wendy Sardo