hi -
white as you may be aware, is not an official colour (
as neither is black, they are both made of raw material derivitives). but that is going off the subject - lol.
cotton in its natural state is an off cream colour. so if cotton during manufacture of clothing, is needed to be white (that is the colour the garment was intended to be), it is bleached to white - and then you have a white garment for wearing, so therefore the more you bleach it - the whiter it gets. but too much bleaching, can weaken the fibres
and if cotton during clothes manufacture is intended to be a colour it is then dyed with a dye to that colour, and then if you bleach that piece of clothing, bleach just takes the dye out (just like resetting the piece of clothing)
so bleach of any kind on white clothing (be careful, as most manmade fibres CANT take chlorine bleach (which is what you used) but on cotton it would not strip the colour out of a pure cotton shirt, as there is no colour to strip in the first place.
if you want to keep clothes clean with bleach (coloureds too), use a laundry colour safe chlorine free bleach, especially designed for clothes, such as ACE bleach.
hope this helps :)