Prepare the fruits: Scoop melon into balls, peel and slice oranges, and halve the grapes, removing seeds if necessary.
Layer the salad: Line a glass bowl with lettuce leaves and arrange melon, oranges, grapes, and walnuts in layers.
Make the dressing: Whisk together yogurt, lemon juice, orange juice, catsup, evaporated milk, salt, and white pepper.
Adjust seasoning: Taste and adjust salt and pepper as needed.
Marinate: Pour the dressing over the salad and let it marinate for 30 minutes.
Toss and serve: Gently toss the salad just before serving to mix the flavors.