Cook 2 pounds small purple potatoes in salted boiling water with 1 teaspoon white vinegar until tender, about 12 minutes; drain. Halve or quarter the potatoes and toss with the juice of 2 limes, 1 teaspoon ground cumin and 1/2 teaspoon each dried oregano and paprika. Add 1 cup diced yellow bell peppers, 1 small minced jalapeno, 1/3 cup chopped cilantro and 1/4 cup olive oil. Season with salt and toss. Top with crumbled queso fresco.