WHISK lime juice, olive oil, garlic, salt, and pepper together in a small bowl. Set aside.
ARRANGE slaw vegetables in a large bowl in color order: red bell pepper, carrot, yellow pepper, snap peas, green cabbage, and purple cabbage. Do not toss yet.
JUST before serving, drizzle dressing over the slaw and toss gently to coat, keeping the colors as visible as possible.
FINISH with chopped scallions and cilantro.
SPOON slaw into butter lettuce cups and serve.