In a pot, melt the butter over medium heat, add the green onion whites, and garlic, and sweat until tender. Add in the saffron threads, and stir. Stir in the rice, ensuring the grains are well coated with the saffron-butter mixture. Pour in the water, and stir to combine. Bring the cooking liquid to a boil, cover, and reduce the heat to a simmer. Cook the rice until the rice is fully cooked, about 25 minutes. Season the rice with salt, to taste.