Keeping white clothes white under the arm pit

I have a real problem with that, and when I got to wash it. My clothes are white but the arm pit area be brownish. And I hate that is there any home remedies to get Rid of it. And the thing at panties, periods make buying new panties SCARY. Because they Sneak up sometimes, Is there a way to get rid of blood stains too? Because I've bought fresh panties and they are messed up.