What is the ruling regarding the appearance of yellow or light brown stains on underwear after exercise or sweating, and what should be done about them? Is it sufficient to wash them in a washing machine if a slight trace of the color remains?

The default state of things is purity until impurity is established with certainty. So, if there is doubt about the impurity of something pure, it remains pure because the original state of affairs persists. Therefore, what is found on underwear after exercise is considered pure, and there is no need to change or wash it, unless impurity is proven. If its removal becomes difficult after washing, it is excused.