For the melon and tomato salad, wash the head of lettuce, shred it and divide it into four salad bowls. Cut the tomatoes into small pieces and discard the goo if necessary.
Cut melon into cubes of about 2 x 2 cm. Spread the melon cubes and tomato pieces on the lettuce leaves.
Mix a marinade from the yogurt, oil, vinegar and ketchup with salt and pink pepper using a blender and spread over the top.