Combine lime juice, cumin, and salt in a medium bowl; whisk to blend.

Slowly add oil, while whisking constantly. Set aside in a medium bowl; mix well. Set aside.

Combine beans, vinegar, and pepper (if desired) in a medium bowl; mix well. Set aside.

Bring water to a boil in a medium saucepan over high heat.

Add quinoa. Reduce heat to medium-low; cook, covered for 10 to 12 minutes, or until all water has been absorbed. Remove from heat. Cool for 15 to 30 minutes. Set aside.

Place cooled quinoa in a large bowl. Fluff with a fork.

Add bell peppers, corn, cilantro, bean mixture, and dressing; toss gently to blend.