"Two witnesses spoke of being physically branded at Chernokozovo. The threat of marking foreheads with green iodine (zelenka) was first heard of in Novye Aldy. Victims detained during the sweep in the village of Shami-Iurt in February 2000 were taken to Tolstoi-Iurt. During transportation, a Russian soldier warned Dashaev and another man named Viskhan that if their noses were marked in red, they were being branded for summary execution. He cautioned that it would be better if they were covered in green iodine. Whether this warning was used as a psychological weapon or had legitimate strategic value is unclear. But both Dashaev and Viskhan were later covered in green iodine and released. The full extent of this practice is unclear." (p.61)
While it seems likely that in the case of the attack on Navalny the iodine was mixed with some other more harmful substance (some reports mention a "green dye attack"), it is interesting to note the apparent origin of this practice.