@morrismm, no worry about disrespect, I was thinking of just buying new shirts, but really wanted to find out why chlorine bleach does not work, so I know for future reference.
@thumper1, once I get to check price of RIT stuff, I probably will end up buying a pack of new undershirt instead, but having the RIT on hand may not be a bad idea in the event I mess up something else in the future! 
@GMTplus7, precisely, that’s what I told my son, they are worn under other shirts, as long as the collar does not stick out of the other shirt, no one can see it! I did use chlorine bleach, undershirts are 100% cotton.