Mix quinoa, pinto beans, kidney beans, corn, red onion, brown rice, red bell pepper, and cilantro together in a glass or plastic container with a lid.
Whisk olive oil, vinegar, chili powder, garlic, salt, black pepper, and cayenne pepper together in a bowl; pour over quinoa mixture and toss to coat. Cover bowl with a lid and refrigerate until flavors blend, at least 2 hours. Stir again before serving.
