Ohh, shirts collars are real challenge, because they quickly absorb sweat and get those yellowish spots that are hard to remove. But you don’t have to use chlorine to make them white.
To clean perfectly the collars of those yellow traces, you have to hand-wash them a bit. More precisely, to apply a natural treatment that will remove any sweat stain, and then wash it as usual.
– baking soda
– a toothbrush
Soak the shirt in lukewarm water, and then sprinkle a consistent layer of baking soda over the yellowish stains (you can sprinkle some powder on the underarm area, too), and wait for half an hour so that baking soda to degrease sweat stains and to remove salt from the fabric. Then wash your shirt like usual, and when you’ll remove it from the washer, you’ll see that those yellow sweat stains are gone from shirts collars.