undershirts are not meant to be seen under dress shirts, period. i don't care what people's opinion on the matter is, you still look like a tool if you do it.
admittedly i used to wear a white crewneck undershirt under my button-downs with the top button undone and the white undershirt revealed, and thought it looked ok. now as my tastes are slightly more refined, i see people walking down the street these days revealing that same look and I just cringe inside. if your gonna rock the undershirt, it better be a white v-neck that doesnt show whatsoever.