Preheat the oven to 375 degrees F and line a baking sheet with parchment paper. Combine the walnuts, melted butter, brown sugar, cumin, paprika, cayenne and 1/4 teaspoon salt in a medium bowl; toss, then spread the nuts on the prepared baking sheet. Bake until the nuts are golden brown, about 10 minutes. Stir, then set aside to cool.
Meanwhile, make the dressing: Whisk the yogurt, lemon juice, honey, rice vinegar and a pinch each of salt and black pepper in a small bowl; set aside.
Trim the tops and bottoms of the grapefruits, then use a paring knife to remove the peel and white pith. Cut between the membranes to release the segments into a large bowl.
Add the spiced walnuts, celery, endive, arugula and chives to the bowl with the grapefruit. Add the yogurt dressing, season with salt and pepper and toss.