In a large bowl, whisk together the yogurt, water, salt, and pepper. Taste and adjust the seasoning, if necessary. Add the grated vegetables and the scallions, but don’t mix them just yet.
In a small skillet, heat the oil over medium-high heat. Add the mustard seeds and curry leaves (be careful, as the mustard seeds will sizzle and pop when they hit the hot oil). Swirl the oil gently until the seeds stop sizzling and the leaves crisp up but are still green, 30 to 45 seconds.
Remove from the heat and add the ginger to the hot oil, stirring for 10 to 15 seconds. Pour the seasoned oil mixture with the seeds, leaves, and ginger over the yogurt and vegetables in the bowl and stir just until blended. (Avoid overmixing or the red beet color will turn the entire dish pink.)