Put the saffron threads and water in a small bowl or cup and let infuse for a few minutes.
Meanwhile, heat the oil in a paella pan or wide, shallow skillet and cook the onion over medium heat, stirring, for 2-3 minutes, or until softened. Add the garlic, bell peppers, and eggplant and cook, stirring frequently for 5 minutes.
Add the rice and cook, stirring constantly, for 1 minute, or until glossy and coated. Pour in the stock and add the tomatoes, saffron and its soaking water, salt, and pepper. Bring to a boil, then reduce the heat and let simmer, shaking the skillet frequently and stirring occasionally, for 15 minutes.
Stir in the mushrooms, green beans, and pinto beans with their can juices. Cook for an additional 10 minutes, then serve immediately.