Whisk together the grape or apple juice, tomato sauce, vinegar, mustard, oil, ginger and salt in a large bowl. Add the onion, stirring to coat in the dressing. Set aside.
Cook the green beans in boiling water for 4 minutes, or until just tender. Drain and refresh in a sieve under cold running water, to cool quickly and retain their bright green colour. Add to the bowl with the onion and toss to combine so they are well coated in the dressing.
Gently fold in the red and white tinned beans and stir everything together. Serve at room temperature or chilled.