Especially in adolescence, the semen secreted by testicles is at its peak, which leads to the residue on underwear of most boys when they get up in the morning.

Basically, it is enough to wash with water, and it is very clean with the general laundry procedure (washing machine+laundry detergent).

For the "color" that can't be washed away, you don't need to care too much, which is harmless.