Combine red and greed babbages, broccoli slaw, carrots, and scallions in a large bowl. Toss to mix well.
Whisk yogurt, mayonnaise,vinegar, lemon juice, and garlic in a medium bowl until smooth. Season to taste with salt and pepper. DO AHEAD: Slaw mixture and dressing can be made 8 hours ahead. Cover seperately and chill.
No more then 20 minutes before serving, cut core from apple and cut apple into matchstick-size pieces. Add apple to slaw mixture and toss to evenly incorporte, Add dress ( adding earlier will result in a soggy slaw ) and toss to evenly coat.
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