1/4 cup warm water
4 tablespoons of baking soad
Mix to form a paste. Run warm water over the collar to moisten it. Using a clean soft toothbrush, gently scrub the area and let soak for 30 minutes. Rinse well with warm water. Laundry as normal.
Baking soda is an alkaline solution that works well to remove acidic sweat stains.