Coat an 8- x 8-inch baking dish with cooking spray. Line it with 2 pieces of parchment paper, leaving a 1-inch overhang on the sides; coat with cooking spray.

Bring sugar, lemon juice, salt, and 1 cup of the water to a boil in a medium saucepan over medium-high. Reduce heat to medium, and simmer, undisturbed, until a candy thermometer registers 230°F, 7 to 8 minutes. (Continue to simmer until mixture reaches 245°F.) Once sugar mixture reaches 230°F, whisk together ½ cup of the cornstarch and 1 cup of the water in a separate medium saucepan. Bring to a boil over medium-high, and cook, whisking constantly, until very thick, 3 to 5 minutes. Remove from heat.

Once the sugar mixture reaches 245°F, stir cornstarch mixture into sugar mixture, and whisk constantly until the mixture looks smooth, about 3 minutes. Return to a boil over medium-high, and cook, undisturbed, until the candy thermometer registers about 250°F, 12 to 14 minutes. Remove from heat.

Meanwhile, stir together gelatin and remaining ¼ cup water. Add to mixture in saucepan, and whisk until completely smooth. Stir in rosewater, orange blossom water, and food coloring. Pour half of the mixture into the prepared baking dish, and spread half of the pistachios over top. Stir remaining pistachios into remaining half of mixture in saucepan; pour over pistachio layer in baking dish. Let stand at room temperature, uncovered, until completely cool and set, at least 12 hours or up to 24 hours.