Drain the soaked black and red beans separately in a strainer. In two medium saucepans, bring 5 cups of water (2 1/2 cups in each) to a boil. Add the black beans to one pot and the red beans to another, reduce the heat under both to medium, and simmer for about 45 minutes, until the beans are tender but not soft.
Drain the beans separately in a colander and set aside, covered. Bring the remaining 3 cups of lightly salted water to a boil in one of the saucepans, and fill a large bowl with cold water and add several ice cubes to it. Cook the string beans for 3 to 4 minutes, depending on their size, until they are tender but still bright green.
Immediately drain them and plunge them into the ice water to retain their green color. Drain and dry on paper towels.
In a medium heavy skillet cook the bacon strips over medium heat until crisp. Remove the bacon with a slotted spoon and drain briefly on paper towels.
In a small bowl, measure 1 tablespoon of the bacon fat and whisk it together with the remaining ingredients (except lemon wedges).
In a large serving bowl, combine all the beans and drizzle the dressing over. Toss gently just to coat all the ingredients evenly, crumble the bacon over the top and serve while still slightly warm, Garnish with lemon wedges.