I've sported the underarmour white tees under khakis. They just look like REALLY white t-shirts. The collar looks the same if you try on a few sizes and see what looks best. I got away with it for awhile until my CO noticed my "super white t-shirt", looked closer and then thought I was wearing a rash guard under my uniform to skip out of work to go surf. Word got around the wardroom, and everytime I walked in any of the spaces I was greeted with "WE WILL PROTECT THIS HOUSE!!!" Love those J.O.'s....(I still wore 'em, they stay white, look normal, are comfortable, and don't stretch out as much as cotton. I wouldn't trust a ship's laundry with them though).
edit: they are kind of pricey, somewhere between 15 and 20 bucks a pop I think, as opposed to the ol' Hanes 3 pack for what, 10 bucks? I bet someone makes a decent generic verision.
all that said, I don't think anyone would care/notice except surfers and the ship nazis, forever scrutining uniforms for "flammable, melt-to-your-skin" stuff.
and yes, before it's posted, I'm SURE it's against some regulation somewhere...I pick my battles...besides, Skipper thought it was cool...