Originally Posted by Rick
Rice contains arsenic at a relatively high level compared to most other foods.
Wikipedia, the source of all reliable information, states, "As arsenic is a natural element in soil, water, and air, the United States Food and Drug Administration (FDA) monitors the levels of arsenic in foods, particularly in rice products used commonly for infant food. While growing, rice plants tend to absorb arsenic more readily than other food crops, requiring expanded testing by the FDA for possible arsenic-related risks associated with rice consumption in the United States. In April 2016, the FDA proposed a limit of 100 parts per billion (ppb) for inorganic arsenic in infant rice cereal and other foods to minimize exposure of infants to arsenic.."
Thus its seems if the key to the rice water stain process is arsenic, where the rice is grown would greatly affect the quality of the stain. So possibly the region in which this technique originated it was highly effective due to very high concentrations of arsenic in the soil. The downside is that the local population's health may have been compromised.