Young ginger (shin-sho) is preferred for this recipe as it is less fibrous, sweeter, and turns pink more easily than mature ginger. If you can only find mature ginger, slice it extremely thin and consider blanching it briefly in boiling water for 1 minute before pickling to soften the fibers.
The pink color comes from the interaction between the ginger and the vinegar. If the color fades over time, add a tiny piece of red food coloring or a slice of red beet to the jar to maintain the vibrant hue.
For a stronger flavor, increase the marinating time to 2 weeks. The ginger will become more tender and the flavors more pronounced.